0001564590-21-008800 | 10-K | XBRL Viewer | Apellis Pharmaceuticals INC

Development Derivative Liability

Common Stock

3. Common Stock

The Company has reserved the following shares of common stock for future issuance (in thousands):

	December 31,
	2020		2019		2018
Shares reserved under 2010 Equity Incentive Plan		—		—		5,013
Shares reserved under 2017 Equity Incentive Plan		8,613		6,319		4,033
Shares reserved under 2017 Employee Stock Purchase Plan		913		972		972
Common stock warrants		—		—		14
Total		9,526		7,291		10,032

Development Derivative Liability

12 Months Ended

Dec. 31, 2020

Research And Development [Abstract]

4. Development Derivative Liability

On February 28, 2019, the Company entered into the SFJ agreement under which SFJ agreed to provide funding to the Company to support the development of pegcetacoplan for the treatment of patients with PNH. Pursuant to the agreement, SFJ paid the Company $60.0 million following the signing of the agreement, and agreed to pay the Company up to an additional $60.0 million in the aggregate in three equal installments upon the achievement of specified development milestones with respect to the Company’s Phase 3 program for pegcetacoplan in PNH and subject to the Company having cash resources at the time sufficient to fund at least 10 months of the Company’s operations.

On June 7, 2019, the Company and SFJ amended the development funding agreement. Under the SFJ amendment, SFJ agreed to make an additional $20.0 million funding payment to the Company to support the development of pegcetacoplan for the treatment of patients with PNH.

In the years ended December 31, 2020 and 2019, the Company received $20.0 million and $120.0 million, respectively, from SFJ, as the Company met milestones as identified in the SFJ Agreement.

Under the SFJ agreement following regulatory approval by the FDA or EMA for the use of pegcetacoplan as a treatment for PNH the Company will be obligated to pay SFJ an initial payment of up to $5.0 million (or a total of $10.0 million if regulatory approval is granted by the FDA and the EMA) and then up to an additional $226.0 million in the aggregate (or up to $452.0 million if regulatory approval is granted by the FDA and the EMA) in six additional annual payments with the majority of the payments being made from the third anniversary to the sixth anniversary of regulatory approval.

The SFJ agreement is presented as a derivative liability on the consolidated balance sheet and is considered a level three derivative and, as such is remeasured each quarter. The liability was initially recorded at the value of the $60.0 million aggregate cash received pursuant to the contractual terms. During the years ended December 31, 2020 and 2019, the Company received an additional $20.0 million and $60.0 million respectively under the SFJ agreement as the Company met additional milestones in the agreement. The derivative liability was remeasured quarterly at fair value, with the total change in fair value of $103.0 million and $14.8 million recorded for the years ended December 31, 2020 and 2019, respectively in loss from remeasurement of development derivative liability on the consolidated income statement. The remeasurement of the development derivative liability resulted in a remeasured fair value of $257.9 million and $134.8 million as of December 31, 2020 and 2019, respectively on the consolidated balance sheet. At December 31, 2020, $4.2 million of the $257.9 million of the development derivative liability fair market value is included in current liabilities.

The following table presents a rollforward of the liability (in thousands):

	For the Year Ended December 31,
	2020		2019
Balance at fair market value, January 1,	$	134,839	$	—
Amounts received under the SFJ agreement and SFJ amendment		20,000		120,000
Loss recorded in loss from remeasurement of development derivative liability		103,029		14,839
Balance at fair market value, December 31,	$	257,868	$	134,839

The derivative is valued using a scenario-based discounted cash flow method, whereby each scenario makes assumptions about the probability and timing of cash flows, and such cash flows are present valued using a risk-adjusted discount rate. The analysis is calibrated such that the value of the derivative as of the date of the SFJ agreement was consistent with an arm’s-length transaction. Key inputs to the level 3 fair value model include (i) the probability and timing of achieving stated development milestones to receive the next tranches of funding, (ii) the probability and timing of achieving FDA and EMA approval, (iii) SFJ’s cost of borrowing (8.0%), and (iv) the Company’s cost of borrowing (12.65%).

SFJ’s implied cost of borrowing was 8.0% and the Company’s implied cost of borrowing was 12.65% as of the reporting date. These implied costs of borrowing were determined assuming the SFJ agreement was initially executed with arm’s-length terms. If the SFJ agreement was instead not determined to be an arm’s-length transaction, then implied discount rates could differ.

Accrued Expenses and Prepaid Assets

12 Months Ended

Dec. 31, 2020

Accrued Expenses And Prepaid Assets [Abstract]

Accrued Expenses and Prepaid Assets

5. Accrued Expenses and Prepaid Assets

Accrued expenses are as follow (in thousands):

	December 31,
	2020		2019
Accrued research and development	$	47,879	$	36,449
Accrued license fee		25,050		—
Accrued payroll liabilities		22,896		11,443
Other		16,110		6,891
Total	$	111,935	$	54,783

Prepaid assets include $8.0 million and $18.1 million of prepaid research and development costs as of December 31, 2020 and 2019, respectively.

Long-term Debt

12 Months Ended

Dec. 31, 2020

Debt Disclosure [Abstract]

Long-term Debt

6. Long-term Debt

Convertible Senior Notes

On September 16, 2019, the Company completed a private offering of the 2019 Convertible Notes with an aggregate principal amount of $220.0 million issued pursuant to an indenture (the “Indenture”) with U.S. Bank National Association, as trustee (the “Trustee”).

The net proceeds from the sale of the 2019 Convertible Notes were approximately $212.9 million after deducting the initial purchasers’ discounts and commissions of $6.6 million and offering expenses of $0.5 million paid by the Company. The Company used $28.4 million of the net proceeds from the sale of the Convertible Notes to pay the cost of the capped call transactions described below.

On May 12,2020, the Company issued the 2020 Convertible Notes with an aggregate principal amount of $300.0 million. The net proceeds from the sale of the 2020 Convertible Notes were approximately $322.9 million after deducting the purchasers’ discounts and commission of $5.7 million and offering expenses of $0.3 million. The Company used $43.1 million of the net proceeds from the sale to pay the cost of the additional capped call transactions in May 2020 described below.

The Convertible Notes are senior unsecured obligations of the Company and bear interest at a rate of 3.5% per year payable semiannually in arrears on March 15 and September 15 of each year, beginning on March 15, 2020. The Convertible Notes will mature on September 15, 2026, unless converted earlier, redeemed or repurchased in accordance with the terms of the Convertible Notes.

The Convertible Notes are convertible into shares of the Company’s common stock at an initial conversion rate of 25.3405 shares per $1,000 principal amount of Convertible Notes (equivalent to an initial conversion price of approximately $39.4625 per share of common stock). The conversion rate is subject to customary anti-dilution adjustments. In addition, following certain events that occur prior to the maturity date or if the Company deliver a notice of redemption, the Company will increase the conversion rate for a holder who elects to convert its Convertible Notes in connection with such corporate event or a notice of redemption, as the case may be, in certain circumstances as provided in the indenture.

Prior to March 15, 2026, the Convertible Notes are convertible only upon the occurrence of certain events. On or after March 15, 2026 until the close of business on the second scheduled trading day immediately preceding the maturity date of the Convertible Notes, holders may convert the Convertible Notes at any time. Upon conversion of the Convertible Notes, the Company will pay or deliver, as the case may be, cash, shares of the Company’s common stock or a combination of cash and shares of common stock, at the Company’s election.

Prior to September 20, 2023, the Company may not redeem the Convertible Notes. The Company may redeem for cash all or a portion of the Convertible Notes, at its option, on or after September 20, 2023 if the last reported sale price of the Company’s common stock has been at least 130% of the conversion price then in effect for at least 20 trading days (whether or not consecutive), including the trading day immediately preceding the date on which the Company provides a notice of redemption, during any 30 consecutive trading day period ending on, and including, the trading day immediately preceding the date on which the Company provides notice of redemption. The redemption price will be equal to 100% of the principal amount of the Convertible Notes to be redeemed, plus accrued and unpaid interest to, but excluding, the redemption date.

If the Company undergoes a “fundamental change,” as defined in the Indenture, prior to maturity, subject to certain conditions, holders may require the Company to repurchase for cash all or any portion of their Convertible Notes at a fundamental change repurchase price equal to 100% of the principal amount of the notes to be repurchased, plus any accrued and unpaid interest to, but excluding, the fundamental change repurchase date.

The Company used an effective interest rate of 10.5% to determine the liability component of the 2019 and 2020 Convertible Notes. This resulted in the recognition of $145.1 million and $204.5 million as the liability component of the 2019 and 2020 Convertible Notes, respectively, and the recognition of the residual amount of $74.9 million and $95.5 million as the debt discount with a corresponding increase to additional paid in capital for the equity component of the 2019 and 2020 Convertible Notes, respectively. The 2020 aggregate debt issuance costs of $6.0 million were allocated to the liability and equity components in the amounts of $3.7 million and $2.3 million, respectively. The 2019 Convertible Notes aggregate debt issuance costs of $7.1 million were allocated to the liability and equity components in the amounts of $4.7 million and $2.4 million, respectively.

Interest expense for the Convertible Notes was $29.9 million for the year ended December 31, 2020 and $4.4 million from inception through December 31, 2019. For the year ended December 31, 2020, interest expense included the amortization of the discount on the Convertible Notes of $15.0 million, accrued semi-annual coupon payable of $14.4 million and amortization of debt issuance costs of $0.5 million. From inception through December 31, 2019, interest expense included amortization of the discount on

the Convertible Notes of $2.1 million, accrued semi-annual coupon payable of $2.2 million and amortization of debt issuance costs of $0.1 million. As of December 31, 2020 and 2019, $7.8 million and $4.6 million, respectively of debt issuance costs was recorded on the consolidated balance sheet as a reduction to the carrying amount of the Convertible Notes.

The aggregate balance of the Convertible Notes, net of unamortized debt issuance costs, as of December 31, 2020 and 2019 was $358.8 million and $142.6 million, respectively. See note 20 “Subsequent events” for discussion on the exchange of $126.1 million of 2019 Convertible Notes to shares of common stock.

Capped Call Transactions

On September 11, 2019, and May 6, 2020 concurrently with the pricing of the 2019 Convertible Notes and the 2020 Convertible Notes, the Company entered into capped call transactions with two counterparties. The capped call transactions are expected generally to reduce the potential dilution to the Company’s common stock upon any conversion of Convertible Notes and/or offset any cash payments the Company is required to make in excess of the principal amount of converted Convertible Notes, as the case may be, in the event that the market price per share of the Company’s common stock, as measured under the terms of the capped call transactions, is greater than the strike price of the capped call transactions, which is initially $39.4625 (the conversion price of the Convertible Notes) and is subject to anti-dilution adjustments substantially similar to those applicable to the conversion rate of such Convertible Notes. If, however, the market price per share of the Company’s common stock, as measured under the terms of the capped call transactions, exceeds the cap price of the capped call transactions, which is initially $63.14 per share, representing a premium of 100% above the last reported sale price of $31.57 per share of its common stock on The Nasdaq Global Select Market on September 11, 2019, there would nevertheless be dilution and/or there would not be an offset of such potential cash payments, in each case, to the extent that such market price exceeds the cap price of the capped call transactions.

Pursuant to ASC 815-40 Derivatives and Hedging, the Company determined that the capped call transactions should be classified as equity instruments and the capped call premium paid in the amount of $28.4 million and $43.1 million were recorded as reductions to additional paid-in capital upon the issuance of the Convertible Notes.

Term Loan Facility

On October 20, 2017, the Company entered into a loan and security agreement with Silicon Valley Bank (“SVB”) to provide for a $20.0 million term loan facility (the “term loan facility”). Borrowings under the term loan facility accrued interest at a floating rate per annum equal to the WSJ prime rate plus 1.50%. Under the agreement, the Company was required to make monthly interest-only payments through November 1, 2019 and to make 24 equal monthly payments of principal, plus accrued interest, thereafter from November 1, 2019 through October 1, 2021, when all unpaid principal and interest would become due and payable.

On March 26, 2019, the Company voluntarily repaid all outstanding amounts due and owed, including applicable termination fees, under the term loan facility. The final payment of $21.8 million totaled per diem interest of $0.1 million and $21.7 million for the outstanding balance of the term loan facility which included (i) a final payment equal to 8% of the original principal amount of the term loan facility of $1.6 million, and (ii) a prepayment fee contractually owed of $0.1 million plus other fees which resulted in a total loss on extinguishment of debt of $1.2 million.

In connection with the Company’s entry into the term loan facility, the Company issued to SVB a warrant to purchase 14,064 shares of the Company’s common stock with an exercise price per share of $5.484. The warrant had a ten-year term and included a put option pursuant to which, in the event of an acquisition, change in control or dissolution or winding up of the Company or the expiration of the warrant, SVB could require the Company to repurchase the warrant for a total aggregate purchase price of $250 thousand. The warrants were exercised in November 2019.

Promissory Note

On October 19, 2017, the Company issued and sold an unsecured promissory note in the principal amount of $7.0 million to Golda Darty Partners S.A. (“GDP”). The promissory note accrued interest at a rate per annum of 8.0%, and was due and payable quarterly in arrears on the 19th day of each April, July, October and January. The promissory note had a maturity date of October 19, 2022 when the $7.0 million would be due and payable in its entirety. The promissory note was contractually subordinated to the Company’s obligations to SFJ under the SFJ agreement.

On September 16, 2019, the Company voluntarily repaid all outstanding amounts due and owed under the promissory note, except for a small amount of interest subsequently repaid. The payment of $7.1 million reflected per diem interest of $0.1 million and $7.0 million for the outstanding principal balance of the promissory note.

In connection with the issuance and sale of the above promissory note, the Company issued to GDP a warrant to purchase 93,764 shares of the Company’s common stock at a price per share of $5.484, which was exercised in whole in October 2017. The

Company recorded the fair value of the warrant in the aggregate amount of $0.4 million as a discount to the promissory note. This amount was being accreted as additional interest expense over the term of the promissory note. Upon the repayment of the promissory note, the Company recorded a loss on extinguishment of debt of $0.3 million due to the remaining discount.

The contractual maturities of the Company’s long-term debt obligations due subsequent to December 31, 2020 consist of the $520.0 Convertible Senior Notes which mature in September 2026.

Leases

12 Months Ended

Dec. 31, 2020

Leases [Abstract]

Leases

7. Leases

On January 1, 2019, the Company adopted ASU 2016-02 Leases (Topic 842) using a modified retrospective method. The Company recognized $5.5 million of lease assets and liabilities. There was no impact to retained earnings upon adoption of Topic 842. The underlying assets of the Company’s leases primarily relate to office space leases, but also include some equipment leases. The Company determines if an arrangement qualifies as a lease at its inception. During 2019, the Company leased additional office space resulting in an initial recognition value of $9.8 million of lease assets and liabilities.

As a practical expedient permitted under Topic 842, the Company has elected to account for the lease and non-lease components as a single lease component for all leases of which it is the lessee. Lease payments, which may include lease and non-lease components, are included in the measurement of the Company’s lease liabilities to the extent that such payments are either fixed amounts or variable amounts that depend on a rate or index as stipulated in the lease contract. When the Company cannot readily determine the rate implicit in the lease, the Company determines its incremental borrowing rate by using the rate of interest that it would have to pay to borrow on a collateralized basis over a similar term, an amount equal to the lease payments in a similar economic environment.

The Company enters into lease agreements with terms generally ranging from 2-7 years. Some of the Company’s lease agreements include Company options to extend the lease on a month-to-month basis or for set periods for up to five years. Many leases also include options to terminate the leases within one year or per other contractual terms. Renewal and termination options were generally not included in the lease term for the Company’s existing operating leases.

As of December 31, 2020 and 2019, all leases were classified as operating lease assets and liabilities. Additional information related to the operating lease assets and liabilities is as follows (in thousands):

	2020			2019
Operating lease assets	$	17,719		$	14,110
Operating lease liabilities	$	18,902		$	14,466
Weighted average remaining term in years		4.66		5.01
Weighted average discount rate used to measure outstanding lease liabilities		7.74	%		8.19	%

For the years ended December 31, 2020 and 2019, the total lease cost for operating lease expense was approximately $4.4 million and $2.4 million, respectively.

Supplemental cash flow information related to operating leases for the years ended December 31, 2020 and 2019 is as follows (in thousands):

	2020		2019
Operating cash flows from operating leases	$	4,732	$	2,013
Operating lease assets obtained in exchange for lease obligations	$	7,237	$	10,201

The maturity of the Company’s operating lease liabilities as of December 31, 2020 are as follows (in thousands):

2021	$	4,962
2022		4,842
2023		4,826
2024		4,114
2025 and thereafter		3,745
Total future minimum lease payments		22,489
Less imputed interest		(3,587	)
Total operating lease liabilities	$	18,902

Marketable Securities

12 Months Ended

Dec. 31, 2020

Investments Debt And Equity Securities [Abstract]

Marketable Securities

8. Marketable Securities

The amortized cost, gross unrealized holding losses and fair value of available-for-sale debt securities by type of security as of December 31, 2020 were as follows (in thousands):

	As of December 31, 2020
	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Fair Value
U.S. Government-related obligations	$	311,877	$	11	$	(19	)	$	311,869

All available-for-sale securities mature in one year or less. The Company did not hold available-for-sale securities as of December 31, 2019.

Other Comprehensive Income and Accumulated Other Comprehensive Income

12 Months Ended

Dec. 31, 2020

Other Comprehensive Income and Accumulated Other Comprehensive Income

9. Other Comprehensive Income and Accumulated Other Comprehensive Income

The following tables summarize the changes in accumulated other comprehensive income/(loss), by component for the years ended December 31, 2020 and 2019 (in thousands):

	Unrealized Gains (Losses) from Marketable Securities			Foreign Currency Translation Adjustment			Total Accumulated Other Comprehensive Income (Loss)
Balances, December 31, 2019	$	—		$	(154	)	$	(154	)
Net other comprehensive income (loss)		(8	)		45			37
Balances, December 31, 2020	$	(8	)	$	(109	)	$	(117	)


	Unrealized Gains (Losses) from Marketable Securities			Foreign Currency Translation Adjustment			Total Accumulated Other Comprehensive Income (Loss)
Balances, December 31, 2018	$	—		$	(123	)	$	(123	)
Net other comprehensive income (loss)		—			(31	)		(31	)
Balances, December 31, 2019	$	—		$	(154	)	$	(154	)

Fair Value Measurements

12 Months Ended

Dec. 31, 2020

Fair Value Disclosures [Abstract]

Fair Value Measurements

10. Fair Value Measurements

Level 1 – Quoted prices in active markets for identical assets or liabilities;

Level 2 – Inputs other than quoted prices included within level 1 that are observable for the asset or liability, either directly or indirectly;

Level 3 – Unobservable inputs that reflect the Company’s own assumptions about the assumptions market participants would use in pricing the asset or liability.

The following table presents the fair value of financial instruments recorded originally at amortized cost or fair value and not re-measured on a recurring basis (in thousands):

		December 31, 2020
Balance Sheet Classification:	Type of Instrument	Level 1		Level 2		Level 3		Total
Financial Assets:
Cash and cash equivalents:	Money market funds	$	427,515	$	—	$	—	$	427,515
	Bank certificates of deposit		43,577		—		—		43,577
Total Financial Assets		$	471,092	$	—	$	—	$	471,092

		December 31, 2019
Balance Sheet Classification:	Type of Instrument	Level 1		Level 2		Level 3		Total
Financial Assets:
Cash and cash equivalents:	Money market funds	$	281,314	$	—	$	—	$	281,314
Total Financial Assets		$	281,314	$	—	$	—	$	281,314

The Company’s Convertible Notes fall into the Level 2 category within the fair value level hierarchy. The fair value was determined using broker quotes in a non-active market for valuation. As of December 31, 2020 and 2019, the debt component of the Company’s Convertible Notes had a fair value of approximately $676.2 million and $149.5 million, respectively. The Convertible Notes accrue a semi-annual coupon at an annual rate of 3.5%, which was included in accrued expenses in the consolidated balance sheet at December 31, 2020 and 2019.

The following table presents the fair value of financial instruments recorded at fair value at inception and remeasured on a recurring basis (in thousands):

		December 31, 2020
Balance Sheet Classification:	Type of Instrument	Level 1		Level 2		Level 3		Total
Financial Assets:
Cash and cash equivalents:	US government obligations	$	89,990	$	—	$	—	$	89,990
Marketable securities:	US government obligations		311,869		—		—		311,869
Total Financial Assets		$	401,859	$	—	$	—	$	401,859

Financial Liabilities:
Development derivative liability	Development derivative liability	$	—	$	—	$	257,868	$	257,868
Total Financial Liabilities		$	—	$	—	$	257,868	$	257,868

		December 31, 2019
Balance Sheet Classification:	Type of Instrument	Level 1		Level 2		Level 3		Total
Financial Liabilities:
Development derivative liability	Development derivative liability	$	—	$	—	$	134,839	$	134,839
Total Financial Liabilities		$	—	$	—	$	134,839	$	134,839

The fair value of the SFJ agreement is presented as a development derivative liability based on level 3 inputs. The derivative is valued using a scenario-based discounted cash flow method, whereby each scenario makes assumptions about the probability and timing of cash flows, and such cash flows are present valued using a risk-adjusted discount rate. The analysis is calibrated such that the value of the derivative as of the date of the SFJ agreement was consistent with an arm’s-length transaction. Key inputs to the level 3 fair value model include (i) the probability and timing of achieving stated development milestones to receive the next tranches of

funding, (ii) the probability and timing of achieving FDA and EMA approval, (iii) SFJ’s cost of borrowing (8.0%), and (iv) the Company’s cost of borrowing (12.65%). A 10% change in the key inputs of i) the probability and timing of payment and ii) the discount rate utilized would result in a change in fair value of the development derivative liability of approximately 5.0% or $12.9 million.

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

Organization Consolidation And Presentation Of Financial Statements [Abstract]

License and Collaboration Agreements

12 Months Ended

Dec. 31, 2020

License and Collaboration Agreements

11. License and Collaboration Agreements

Sobi Transaction

On October 27, 2020, the Company and its subsidiaries Apellis Switzerland GmbH and APL DEL Holdings, LLC entered into a Collaboration and License Agreement with Sobi, concerning the development and commercialization of pegcetacoplan and specified other structurally and functionally similar compstatin analogues or derivatives for use systemically or for local non-ophthalmological administration (collectively referred to as the “Licensed Products”).

Under the collaboration agreement, the Company granted Sobi an exclusive (subject to certain retained rights of the Company), sublicensable license of certain patent rights and know-how to develop and commercialize Licensed Products in all countries outside of the United States. The Company retains the right to commercialize Licensed Products in the United States, and, subject to specified limitations, to develop Licensed Products worldwide for commercialization in the United States.

Under the collaboration agreement, the Company and Sobi have agreed to collaborate to develop Licensed Products for the treatment of paroxysmal nocturnal hemoglobinuria, cold agglutinin disease, hematopoietic stem cell transplantation-associated thrombotic microangiopathy, C3 glomerulopathy and immune complex membranoproliferative glomerulonephritis, and amyotrophic lateral sclerosis, and any other indications subsequently agreed upon by the parties, for commercialization by or on behalf of the Company in the United States and by or on behalf of Sobi outside of the United States. If the parties do not agree to jointly pursue any development activities for the Licensed Products (whether for an Initial Indication or otherwise), the party proposing to pursue such activities may conduct such activities at its sole expense (with the non-proposing party having the right to obtain rights to the data generated by such development activities by paying a specified percentage of that expense), subject to agreed-upon exceptions that limit each party’s unilateral development rights.

The initial development plan sets forth the initial development activities to be conducted by each of the Company and Sobi, with the Company bearing all costs incurred in conducting the activities set forth in such initial development plan, as well as certain specified additional costs that are not included in the initial development plan that may be incurred by the parties in developing Licensed Products for paroxysmal nocturnal hemoglobinuria in the European Union and the United Kingdom. The Company and Sobi will form several governance committees to oversee the development and manufacture, and to review and discuss the commercialization, of Licensed Products.

The Company shall supply Licensed Products to Sobi for development and for commercialization outside of the United States in accordance with a supply agreement to be negotiated by the parties. The collaboration agreement grants Sobi the right to perform or have performed drug product manufacturing of Licensed Products for development and for commercialization outside the United States and to manufacture or have manufactured drug substance under certain circumstances.

Accounting Analysis

The Company has determined that the agreement is within the scope of ASC 808 as a contractual arrangement that involves a joint operating activity whereby both parties are (i) active participants in the activity and (ii) exposed to certain significant risks and rewards dependent on the commercial success of the activity. ASC Topic 808 does not address measurement or recognition matters

but allows for analogizing to ASC 606. Pursuant to ASC 606, the Company performed the following five steps: (i) identified the contract(s) with a customer; (ii) identified the performance obligations in the contract; (iii) determined the transaction price; (iv) allocated the transaction price to the performance obligations in the contract; and (v) recognized revenue when (or as) the entity satisfies a performance obligation.

The Company identified the following material distinct promises under the Sobi Agreement: (1) licenses to develop and commercialize pegcetacoplan or, Licenses to IP, and (2) performance of research and development services. The Company determined the promises to be distinct because Sobi can benefit from each of the license and the development services on their own or with readily available services. The Company could have provided the license without any development services and Sobi would have been able to benefit from it by obtaining development services from another provider as the Licensed Products are at a more mature stage in their life cycle.

Under the agreement, Sobi agreed to pay the Company

a fixed amount of $250.0 million in an upfront payment in November 2020;

ii)

a fixed amount of an additional $80.0 million in development reimbursements, payable yearly in four tranches in amounts determined based upon actual expenses incurred by the Company;

iii)

up to an aggregate of $915.0 million upon the achievement of specified one-time regulatory and commercial milestone events; and

iv)

tiered, double-digit royalties, ranging from high teens to high twenties, on sales of Licensed Products outside of the United States, subject to customary deductions and third-party payment obligations.

At contract inception, the $250.0 million non-refundable payment and the $80.0 million reimbursements were fixed proceeds. The Company evaluated whether Sobi is a customer for either of the distinct promises in the agreement. Under the Licenses to IP, the Company determined that Sobi is a customer as the know-how provided and the right granted by the Company to Sobi are outputs of the Company’s business activities for which the Company will receive consideration. With respect to research and development activity, management determined that there is no vendor relationship as performing research and development activities for others is not a part of the Company’s ongoing central operations. Based upon the evaluation of the relative fair values, the Company allocated the purchase price of $250.0 million and the related milestones and royalties to the license of IP and $80.0 million to performance of research and development activities.

The milestone and royalty payments are subject to activities outside the control of the Company. Per ASC 606, the Company considers this to be a customer/ vendor relationship, therefore, the Company will include the regulatory milestone payments in the total transaction price when it is probable that a significant reversal of revenue would not occur in a future period. The Company will recognize commercial milestone and royalty revenue at the later of (i) when the related sales occur or (ii) when the performance obligation to which the commercial milestone or royalty has been allocated has been satisfied. In case of commercial milestone or royalty payments, the Company will recognize revenue in the same period that the sales are completed for which the Company is contractually entitled to the milestone or percentage-based royalty payment. To date, the Company has not recognized any commercial milestone or royalty revenue resulting from any of our licensing arrangements. Management will periodically assess the elements of the contract and re-evaluate revenue recognition as necessary.

Pursuant to ASC 606, the Company has recognized the $250.0 million in revenue as this is the amount allocated to the license. The $80.0 million reimbursement for research and development activities does not constitute a customer/vendor relationship and thus is not in the scope of ASC 606. As ASC 808 does not include recognition guidance, the Company has established an accounting policy to recognize the payments under the reimbursement as a receivable on the balance sheet in an amount that is to be reimbursed based upon expense incurred by the Company, with a contra- research and development expense recognized in the statement of operations, over time as the expenses are incurred.

Under the Sobi collaboration agreement, for the year ended December 31, 2020, the Company recognized $250.0 million of licensing revenue in the consolidated statement of operations. For the year ended December 31, 2020, the Company also recognized in the consolidated statement of operations $43.0 million of contra research and development expense relative to the probable amount to be reimbursed under the $80.0 million for research and development. The Company recorded a corresponding receivable of $43.0 million on the consolidated balance sheet, with $25.0 million and $18.0 million in current and long-term assets, respectively, as of December 31, 2020.

Other agreements

In addition, to the Sobi collaboration agreement, during the year ended December 31, 2020, we entered into two different agreements with third parties to provide APL-9 for use in certain research projects for which $0.6 million was recognized in revenue in the consolidated statement of operations as of December 31, 2020.

License Agreements

12 Months Ended

Dec. 31, 2020

Organization Consolidation And Presentation Of Financial Statements [Abstract]

License Agreements

12. License Agreements

In connection with its purchase of assets from Potentia in September 2014, the Company became party to a license agreement with the Trustees of the University of Pennsylvania (“Penn”) as a result of an agreement to purchase substantially all the assets of Potentia Pharmaceuticals, Inc, for an exclusive, worldwide license to specified patent rights. The Company is required to pay annual maintenance fees of $0.1 million until the first sale of a licensed product. The Company is also required to make milestone payments aggregating up to $3.2 million based upon the achievement of specified development and regulatory milestones and up to $5.0 million based upon the achievement of specified annual sales milestones with respect to each licensed product, and to pay low single-digit royalties based on net sales of each licensed product and with minimum quarterly royalty thresholds. In addition, the Company is obligated to pay a specified portion of income it receives from sublicensees.

In addition, the Company is also party to a license agreement with Penn for an exclusive, worldwide license to specified patent rights for the development and commercialization of products in fields of use, as defined therein. The Company is required to pay annual maintenance fees of $0.1 million until the first sale of a licensed product. The Company is required to make milestone payments aggregating up to $1.7 million, based upon the achievement of development and regulatory approval milestones, and up to $2.5 million, based upon the achievement of annual sales milestones with respect to each of the first two licensed products. The license agreement also requires the Company to pay low single digit royalties based on net sales of each licensed product, subject to minimum quarterly royalty thresholds. In addition, the Company is obligated to pay a specified portion of income it receives from sublicensees.

In 2018, the Company made payments to Penn of $1.0 million net of a credit for the annual license maintenance payment, for the achievement of milestones under these agreements. In 2020, the Company owed payments to of $25.0 million for royalty expense related to the Sobi Agreement and another licensing transaction. This was paid in early January 2021.

In addition to the license agreement with Penn, the Company contracts to conduct research and development activities with third parties. Certain of these contracts commit the Company to pay future milestone payments up to $15.0 million or to pay royalty fees ranging from 3-6% if any of the research results in regulatory approval or commercial revenue for a product.

401(k) Profit Sharing Plan and Trust

12 Months Ended

Dec. 31, 2020

Compensation And Retirement Disclosure [Abstract]

401(k) Profit Sharing Plan and Trust

13. 401(k) Profit Sharing Plan and Trust

In July 2010, the Company adopted an employee profit-sharing plan (the “401(k) Plan”), qualified under Section 401(k) of the Internal Revenue Code (the “IRC”). All of the Company’s full-time employees who have attained the age of 21 are eligible to participate in the 401(k) Plan immediately upon employment. Pursuant to the 401(k) Plan, employees may elect to reduce their current compensation by up to the statutorily prescribed annual limit and have the amount of the reduction contributed to the 401(k) Plan. In 2020, 2019 and 2018, the Company recorded $2.1 million, $0.8 million and $0.3 million respectively, for employer contributions made to the 401(k) Plan.

Income Taxes

12 Months Ended

Dec. 31, 2020

Income Tax Disclosure [Abstract]

Income Taxes

14. Income Taxes

The components of loss from continuing operations before provision for income taxes are as follows (in thousands):

	Year Ended December 31,
	2020			2019			2018
Domestic	$	(118,839	)	$	(297,127	)	$	(122,900	)
Foreign		(224,190	)		(7,580	)		(4,602	)
Total	$	(343,029	)	$	(304,707	)	$	(127,502	)

The provision for income taxes for the year ended December 31, 2020 is as follows (in thousands):

	Year Ended December 31, 2020
Current income tax expense:
U.S. Federal	$	—
U.S. State and Local		57
Foreign		1,788
Total current income tax expense		1,845
Deferred income tax expense:
U.S. Federal		—
U.S. State and Local		—
Foreign		—
Total deferred income tax expense		—
Total tax expense	$	1,845

For the years ended December 31, 2019 and 2018, the Company did not report any current or deferred income tax expense or benefit.

The Company’s effective income tax provision differs from the amount calculated using the statutory U.S. federal income tax rate, principally due to the following (in thousands):

	Year Ended December
	2020						2019						2018
	Amount			Percentage of income before income taxes			Amount			Percentage of income before income taxes			Amount			Percentage of income before income taxes
Statutory U.S. federal income tax	$	(72,036	)		21.0	%	$	(63,988	)		21.0	%	$	(26,775	)		21.0	%
Foreign tax rate differential		22,760			(6.6	)		—			—			—			—
State income taxes, net of federal benefit		(14,107	)		4.1			(16,424	)		5.4			(6,398	)		5.0
Change in valuation allowances		240,065			(69.9	)		90,300			(29.6	)		38,157			(29.9	)
Intellectual property transfer		(162,000	)		47.2			—			—			—			—
Tax credits		(11,696	)		3.4			(6,113	)		2.0			(6,149	)		4.8
Change in state apportionment, Kentucky tax reform and other		93			—			(17	)		—			873			(0.7	)
Permanent and other including share based payments excess deductions		(1,234	)		0.3			(3,758	)		1.2			292			(0.2	)
Effective income tax provision	$	1,845			(0.5	)	$	—			—	%	$	—			—	%

The Company’s effective rate for the year ended December 31, 2020 compared to the year ended December 31, 2019 increased primarily as a result of subsidiary operations in foreign jurisdictions and state income taxes.

During the year ended December 31, 2020, in connection with a corporate reorganization, the Company transferred intellectual property between tax jurisdictions, resulting in deferred tax benefit on basis difference on our intangible assets. The additional deferred tax asset was offset by valuation allowance, resulting in no net impact on our effective tax rate

The Company accounts for income taxes in accordance with ASC Topic 740. Deferred income tax assets and liabilities are determined based upon temporary differences between the financial reporting and tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse.

The following table presents the principal components of the Company’s deferred tax assets and liabilities (in thousands):

	December 31,
	2020			2019
Deferred tax assets:
Intangible assets	$	162,011		$	16
Share-based compensation		11,827			6,625
Deferred interest expense		—			—
Contribution carryforwards		—			41
Net operating loss carryforwards		122,731			102,623
Research and development credits		24,992			14,476
Orphan drug credits		14,372			9,380
Development derivative liability		68,883			35,875
Accruals		4,520			2,259
Other		207			11
Total deferred tax assets		409,543			171,306
Deferred tax liabilities:
Fixed assets		(23	)		—
Convertible debt		(10,473	)		(11,480	)
481(a) adjustment		(1,935	)		(2,891	)
Total deferred tax liabilities		(12,431	)		(14,371	)
Net deferred tax assets before allowance:		397,112			156,935
Less valuation allowance		(397,112	)		(156,935	)
Net deferred tax assets	$	—		$	—

ASC Topic 740 requires a valuation allowance to reduce the deferred tax assets reported if, based on the weight of available evidence, it is more likely than not that some portion or all of the deferred tax assets will not be realized. After consideration of all the evidence, both positive and negative, the Company has recorded full valuation allowances against its domestic and foreign deferred tax assets at December 31, 2020, because management has determined that is it more likely than not that these assets will not be realized. The valuation allowance increased by $240.2 million from December 31, 2019 to December 31, 2020, primarily due to increases in operating losses, development derivative liability, and the step-up in fair value of intellectual property transfer.

At December 31, 2020, the Company had approximately $358.2 million and $405.0 million of Federal and state net operating loss carryforward, respectively. At December 31, 2019, the Company had approximately $392.9 million and $358.3 million of Federal and state net operating loss carryforwards, respectively. The Company also had federal and state research and development tax credit carryforwards $34.2 million and $6.5 million, respectively of as of December 31, 2020. Federal net operating loss carryforward in the amount of $276.9 million may be carried forward indefinitely. The remaining federal and state net operating loss, research and development tax credit carryforwards begin to expire in 2025. The Company also has foreign net operating loss carryforwards of $240.9 million which will begin to expire in 2026.

Under the provisions of the IRC, the net operating loss (“NOL”), and tax credit carryforwards are subject to review and possible adjustment by the Internal Revenue Service and state tax authorities. NOL and tax credit carryforwards may become subject to an annual limitation in the event of certain cumulative changes in the ownership interest of significant stockholders over a three-year period in excess of 50%, as defined under Sections 382 and 383 of the IRC, respectively, as well as similar state provisions. This could limit the amount of tax attributes that can be utilized annually to offset future taxable income or tax liabilities. The amount of the annual limitation is determined based on the value of the Company immediately prior to the ownership change. Subsequent ownership changes may further affect the limitation in future years. The Company has completed several financings since its inception that it believes may have resulted in a change in control as defined by Sections 382 and 383 of the IRC.

The Company does not have any unrecognized tax benefits during any periods presented and does not expect this to significantly change in the next twelve months. There were no interest and penalties recorded in the statement of operations during any period and no amounts accrued for interest and penalties at December 31, 2020 or 2019.

The Company and its subsidiaries file income tax returns in the United States, as well as various state and foreign jurisdictions. Generally, the tax years 2017 through 2019 remain open and subject to examination by the major taxing jurisdictions to which the

Company is subject. To the extent the Company has tax attribute carryforwards, the tax years in which the attribute was generated may still be adjusted upon examination by the Internal Revenue Service, or state or foreign tax authorities, to the extent utilized in a future period.

The entire disclosure for income taxes. Disclosures may include net deferred tax liability or asset recognized in an enterprise's statement of financial position, net change during the year in the total valuation allowance, approximate tax effect of each type of temporary difference and carryforward that gives rise to a significant portion of deferred tax liabilities and deferred tax assets, utilization of a tax carryback, and tax uncertainties information.

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

12 Months Ended

Dec. 31, 2020

Commitments and Contingencies

15. Commitments and Contingencies

The Company has entered into a leasing agreement to amend the current office space leases in Waltham, Massachusetts (the “Amended Waltham office space lease”). The Amended Waltham office space lease adds additional office space in Waltham, Massachusetts with a commencement date of January 1, 2021. The Amended Waltham office space lease amends all prior leasing arrangements for the Waltham office space, has a term of 72 months, and includes leasing an additional 16,401 square feet of office space for a total of 77,818 leased square feet of office space. The Amended Waltham office space lease provides for initial monthly lease payments of $0.3 million per month which is approximately a 20% increase to the existing per month rent payment. The base rent payable over the lease period is $19.4 million.

The Company contracts to conduct research and development activities with third parties. The scope of the services under the research and development contracts can be modified and the contracts cancelled by the Company upon written notice. In some instances, the contracts may be cancelled by the third party upon written notice. If the Company were to cancel these contracts as of December 31, 2019, the Company would be required to pay certain termination costs and other fees of approximately $2.8 million that would be incurred in future periods.

The Company has certain non-cancelable purchase obligations related to the manufacturing of drug substance and drug product, primarily with Bachem Americas, Inc, and Bachem AG, collectively (“Bachem”) for the drug substance for the finished dosage form of pegcetacoplan. As of December 31, 2020, the Company has non-cancellable purchase commitments for 2021 with Bachem in the amount of approximately $13.7 million.

Following regulatory approval by the FDA or EMA of pegcetacoplan for the treatment of PNH, the Company has certain payment and other obligations under the SFJ agreement, which are discussed above in Note 5.

Indemnifications—In the ordinary course of business, the Company enters into agreements that may include indemnification provisions. Pursuant to such agreements, the Company may indemnify, hold harmless and defend indemnified parties for losses suffered or incurred by the indemnified party. Some of the provisions will limit losses to those arising from third-party actions. In some cases, the indemnification will continue after the termination of the agreement. The maximum potential amount of future payments the Company could be required to make under these provisions is not determinable. The Company has not incurred any cost to defend lawsuits or settle claims related to these indemnification provisions.

Legal—During the normal course of business, the Company may be a party to legal claims that may not be covered by insurance. Management does not believe that any such claims would have a material impact on the Company’s consolidated financial statements.

Equity Incentive Plans

12 Months Ended

Dec. 31, 2020

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Equity Incentive Plans

16. Equity Incentive Plans

Share-based Compensation

The Company’s Board of Directors adopted, and its stockholders approved, an equity incentive plan in 2010 (as amended, the “2010 Plan”). The Board of Directors and stockholders amended the 2010 Plan in August 2017 to increase the number of shares of common stock reserved for issuance thereunder to 6,188,466. The 2010 Plan allowed for the grant of incentive stock options and non-qualified stock options to purchase common stock for employees, directors and consultants under terms and conditions established by the Board of Directors. Incentive stock options and nonqualified stock options were granted at exercise prices that were no less than 100% of the estimated fair value per share of the common stock on the date of grant. If an individual owns capital stock representing more than 10% of the voting shares, the price of each share was at least 110% of the fair value on the date of grant. The Board of Directors determined the fair value of common stock with the assistance of a third-party specialist. Options expire 10 years from the issuance date. Following the adoption of the 2017 Stock Incentive Plan, the Company no longer grants stock options or other awards under the 2010 Plan.

In October 2017, the Company’s Board of Directors adopted, and its stockholders approved, the 2017 Stock Incentive Plan (the “2017 Plan”), which became effective on November 8, 2017. The 2017 Plan provides for the grant of incentive stock options, non-statutory stock options, stock appreciation rights, awards of restricted stock, restricted stock units and other stock-based awards. The number of shares of common stock reserved for issuance under the 2017 plan is the sum of (i) 1,359,587 shares of common stock, plus (ii) an additional number of shares of common stock equal to the sum of (a) the number of shares of common stock reserved for

issuance under the 2010 equity incentive plan that remained available for future issuance immediately prior to the effectiveness of the 2017 Plan, which was 299,568 shares, and (b) the number of shares of common stock subject to outstanding awards under the 2010 equity incentive plan upon effectiveness of the 2017 plan that expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased by us at their original issuance price pursuant to a contractual repurchase right plus (iii) an annual increase, to be added the first day of each fiscal year, beginning with the fiscal year ending December 31, 2018 and continuing until, and including, the fiscal year ending December 31, 2027, equal to the lowest of 4,219,409 shares of common stock, 4.0% of the number of shares of common stock outstanding on the first day of the fiscal year and an amount determined by the board of directors.

As of December 31, 2020, a total of 8,613,049 shares of common stock were reserved for issuance under the 2017 Plan. In January 2021, the shares available for future issuance under the 2017 plan were increased by 3,045,189 shares pursuant to the annual increase described above.

Additionally, during 2019 and thereafter, the Company has granted equity awards as equity inducement awards material to entry into employment with the Company to certain newly hired employees outside of the Company’s existing plans in accordance with Nasdaq listing rule 5635(c)(4). In February 2020 the Board of Directors adopted the 2020 Inducement Stock Incentive Plan (the “2020 Plan”), which permitted the Company to grant equity awards to newly hired employees in accordance with Nasdaq listing rule 5635(c)(4). The aggregate number of shares reserved for issuance under the 2020 Plan was initially 750,000 shares but was increased to 1,050,000 shares in January 2021.

In October 2017, the Company’s board of directors adopted and the Company’s stockholders approved the 2017 Employee Stock Purchase Plan (“ESPP”), which became effective upon the IPO and provides participating employees with the opportunity to purchase up to an aggregate of 468,823 shares of common stock. The number of shares of common stock reserved for issuance under the 2017 ESPP will automatically increase on the first day of each fiscal year, beginning with the fiscal year ending December 31, 2018 and continuing until, and including, the fiscal year ending December 31, 2027, equal to the lowest of (i) 937,646 shares of common stock, (ii) 1.0% of the number of shares of common stock outstanding on the first day of the fiscal year and (iii) an amount determined by the board of directors. The board of directors initiated the first offering under ESPP in October 2019.

Total share-based compensation expense related to the various plans during the years ended was as follows (in thousands):

	Year Ended December 31,
	2020		2019		2018
Research and development	$	21,381	$	10,683	$	3,559
General and administrative		23,995		10,461		4,174
Total share-based compensation expense	$	45,376	$	21,144	$	7,733

Stock Options—Options granted generally vest over 48 months. Options granted to employees on or after December 5, 2013 generally vest in installments of (i) 25% at the one-year anniversary and (ii) in either 36 equal monthly or 12 equal quarterly installments beginning in the thirteenth month after the initial vesting commencement date (as defined) subject to the employee’s continuous service with the Company. Options granted before December 5, 2013 vest over four years in equal annual installments of 25% at each anniversary of the grant date.

Under the Executive Separation Benefits and Retention Plan and by resolutions adopted by the Compensation Committee in October 2019, the stock options granted to the Company’s executives and employees will become fully vested upon the occurrence of a change in control, as defined in the Executive Separation Benefits and Retention Plan, if such executive or employee is terminated without cause or resigns for good reason within 12 months after such change in control.

The following table summarizes the Company’s stock option activity:

				Weighted -		Weighted -
				Average		Average		Aggregate
				Exercise		Contractual		Intrinsic
	Shares			Price		Life		Value
	(in thousands)			Per Share		(in years)		(in thousands)
Outstanding, December 31, 2019		10,855		$	13.52
Granted		2,884			37.93
Exercised		(1,208	)		7.80
Forfeited		(795	)		28.03
Outstanding, December 31, 2020		11,736		$	19.13
Options exercisable, December 31, 2020		6,062		$	10.96		6.22	$	280,290
Expected to vest, December 31, 2020		5,674		$	27.85		8.62	$	166,527

The aggregate intrinsic values of options outstanding, exercisable, vested and expected to vest were calculated as the difference between the exercise price of the options and the fair value of the common stock as of December 31, 2020.

During the years ended December 31, 2020, 2019 and 2018, the Company granted stock options to purchase an aggregate of 2.9 million, 4.4 million and 2.1 million shares of its common stock, respectively with weighted average grant date fair values of $27.52, $17.31 and $13.13, respectively.

The aggregate intrinsic value of options exercised during the years ended December 31, 2020, 2019 and 2018 were $38.0 million, $15.7 million, and $5.0 million respectively calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock for the options on the respective date of exercise.

The fair market value of options vested during the years ended December 2020, 2019 and 2018 was $33.6 million, $12.9 million and $5.1 million, respectively.

At December 31, 2020, unrecognized compensation expense related to unvested options, was $100.1 million, which the Company expects to recognize over an estimated weighted-average period of 2.84 years.

The assumptions used in the Black-Scholes model to estimate the grant date fair value are as follows:

	Year Ended December 31,
	2020	2019	2018
Risk-free interest rate	0.32 - 1.76%	1.42 - 2.56%	2.68 - 2.99%
Dividend yield	0%	0%	0%
Volatility	84.4 - 87.8%	102.6 - 111.2%	87.0 - 110.9%
Expected terms (years)	5.31 - 6.08	5.31 - 6.08	5.94 - 6.25

Restricted Stock Units— The fair value of RSU’s is estimated based upon the closing market price of the Company’s common stock on the date of grant. RSUs generally vest annually over a four-year period:

	Number of Stock Units (in thousands)			Weighted Average Grant Date Fair Value Per Share
Unvested Balance at December 31, 2019		—		$	—
Granted		546			38.97
Vested		—			—
Forfeited		(44	)		40.43
Unvested Balance at December 31, 2020		502			38.84

At December 31, 2020, there was approximately $16.2 million of related unrecognized compensation cost which the Company expects to recognize over a remaining weighted average period of 3.4 years.

Employee Stock Purchase Plan—At December 31, 2020, 913,226 shares of common stock remained available for issuance pursuant to the ESPP. Eligible employees who elect to participate in an offering under the ESPP may have up to 15 percent of their earnings withheld, subject to certain limitations, to purchase shares of common stock pursuant to the ESPP. The price of common stock purchased under the ESPP is equal to 85 percent of the lower of the fair market value of the common stock at the commencement date of each offering period or the relevant purchase date. During the year ended December 31, 2020, a total of 58,938 shares of common stock were issued under the ESPP at average per share price of $26.71. During the year ended December 31, 2020, the Company recorded cash received from the issuance of stock to the ESPP of $1.6 million and recorded $0.8 million of stock-based compensation expense related to the ESPP.

Net Loss per Common Share

12 Months Ended

Dec. 31, 2020

Earnings Per Share [Abstract]

Net Loss per Common Share

17. Net Loss per Common Share

The following table presents the calculation of basic and diluted net loss per common share (amounts in thousands except per share amounts):

	Year Ended December 31,
	2020			2019			2018
Numerator:
Net loss	$	(344,874	)	$	(304,707	)	$	(127,502	)
Denominator:
Weighted-average number of common shares used in net loss per common share -- basic and diluted		75,163			62,229			54,396
Net loss per common share -- basic and diluted	$	(4.59	)	$	(4.90	)	$	(2.34	)

Shares outstanding presented below were excluded from the calculation of diluted net loss per share, prior to the use of the treasury stock method, as their effect is anti-dilutive (in thousands):

	Year Ended December 31,
	2020		2019		2018
Convertible notes		13,177		—		—
Common stock under option		11,736		10,854		7,498
Restricted stock units		502		—		—
Common stock warrants		—		—		14
Total		25,415		10,854		7,512

Related Party Transaction

12 Months Ended

Dec. 31, 2020

Related Party Transaction

18. Related Party Transaction

Effective as of May 1, 2018, the Company entered into a subscription license agreement and a services agreement with Revon Systems, Inc. (“Revon”). Under the subscription license agreement, Revon granted the Company an exclusive license to use the Revon software platform and applications for any purpose with respect to the Company's programs in age-related macular degeneration, hemolytic diseases and complement-dependent nephropathies for an annual license fee of $0.2 million and an option to obtain a perpetual, exclusive license thereafter for $0.4 million. Under the services agreement, Revon provided development services with respect to the Revon software to the Company for $0.3 million during the first year. Prior to the acquisition of Revon by an unrelated third party in July 2019, the Company paid the remainder of the annual license fee due for the second year, exercised the option for the perpetual license and discontinued the services agreement. For the year ended December 31, 2019, the Company paid Revon a total of $0.7 million. There were no payments under the agreement during the year ended December 31, 2020.

Each of Cedric Francois, the Company’s chief executive officer, Pascal Deschatelets, the Company’s chief operating officer, and Alec Machiels, a member of the board of directors, was an affiliate of Revon. The Board approved the Revon agreements after review by a subcommittee of the disinterested members of the Board and determination by the full Board that the terms of the Revon agreements were fair, reasonable and in the best interests of the Company. The exercise of the option for the perpetual license was approved in accordance with the Company’s related party transaction policy.

Selected Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2020

Quarterly Financial Information Disclosure [Abstract]

Selected Quarterly Financial Data (Unaudited)

19. Selected Quarterly Financial Data (Unaudited)

The following interim financial information presents the Company’s 2020 and 2019 results of operations on a quarterly basis (amounts in thousands except per share amounts):

	Quarter Ended
	March 31,			June 30,			September 30,			December 31,
	2020			2020			2020			2020
Operating income/ (loss)	$	(98,786	)	$	(115,508	)	$	(129,552	)	$	130,120
Net income/(loss)		(168,822	)		(118,617	)		(135,700	)		78,265
Net income/(loss) per common share, basic (1)		(2.29	)		(1.57	)		(1.79	)		1.03
Net income/(loss) per common share, diluted (1)		(2.29	)		(1.57	)		(1.79	)		0.93

	Quarter Ended
	March 31,			June 30,			September 30,			December 31,
	2019			2019			2019			2019
Operating Loss	$	(48,651	)	$	(63,476	)	$	(69,948	)	$	(105,940	)
Net Loss		(50,574	)		(71,090	)		(69,825	)		(113,218	)
Net Loss per common share, basic and diluted (1)		(0.87	)		(1.12	)		(1.10	)		(1.77	)

(1)

The sum of the four quarters of earnings per share for 2020 and 2019 may not add to the full year earnings per share amount due to rounding and/or the use of quarter-to-date weighted average shares to calculate the earnings per share amount in each respective quarter. For the quarter ended December 31, 2020, diluted net income per common share was calculated using the if-converted method, which adjusts both the numerator by removing the interest expense associated with the Convertible Notes and the denominator by increasing the share count.

Subsequent Events

12 Months Ended

Dec. 31, 2020

Subsequent Events [Abstract]

Subsequent Events

20. Subsequent Events

On January 6, 2021, the Company entered into separate, privately negotiated exchange agreements with certain holders of its 2019 Convertible Notes. Under the terms of these exchange agreements, the holders exchanged approximately $126.1 million in aggregate principal amount of 2019 Convertible Notes held by them for an aggregate of 3,906,869 shares of common stock issued by the Company. The exchange transactions closed in January 2021.

Summary of Significant Accounting Policies (Policies)	12 Months Ended
Summary of Significant Accounting Policies (Policies)	Dec. 31, 2020
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries, Apellis Australia Pty Ltd, Apellis Bermuda Limited, Apellis Germany GmbH, Apellis Ireland Ltd, Apellis Netherlands B.V., Apellis Switzerland GmbH, Apellis UK Limited, APL DEL Holdings LLC, APL Sales Corp I, LLC, APL PRG I, Corp. and Apellis MA Securities Corp. All intercompany balances and transactions have been eliminated in consolidation. The consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and following the requirements of the Securities and Exchange Commission (the “SEC”).
Licensing and Collaboration Revenue	Licensing and Collaboration Revenue The Company analyzes license and collaboration arrangements pursuant to FASB ASC Topic 808, Collaborative Arrangement Guidance and Considerations, (“ASC 808”) to assess whether such arrangements, or transactions between arrangement participants, involve joint operating activities performed by parties that are both active participants in the activities and exposed to significant risks and rewards dependent on the commercial success of such activities or are more akin to a vendor-customer relationship. In making this evaluation, the Company considers whether the activities of the collaboration are considered to be distinct and deemed to be within the scope of the collaborative arrangement guidance or if they are more reflective of a vendor-customer relationship and, therefore, within the scope of FASB ASC Topic 606, Revenue from Contracts with Customer, (“ASC 606”). This assessment is performed throughout the life of the arrangement based on changes in the responsibilities of all parties in the arrangement. For elements of collaboration arrangements that are not accounted for pursuant to guidance in ASC 606, an appropriate recognition method is determined and applied consistently, generally by analogy to the revenue from contracts with customers guidance. Amounts related to transactions with a counterparty in a collaborative arrangement that is not a customer are presented as collaboration revenue and in a separate line item from revenue recognized from contracts with customers, if any, in our consolidated statements of operations. Pursuant to ASC 606, for arrangements or transactions between arrangement participants determined to be within the scope of the contracts with customers guidance, the Company performs the following steps to determine the appropriate amount of revenue to be recognized as we fulfill our obligations: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations based on estimated selling prices; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. We evaluate the performance obligations promised in the contract that are based on goods and services that will be transferred to the customer and determine whether those obligations are both (i) capable of being distinct and (ii) distinct in the context of the contract. Goods or services that meet these criteria are considered distinct performance obligations. The Company estimates the transaction price based on the amount expected to be received for transferring the promised goods or services in the contract. The consideration may include fixed consideration or variable consideration. At the inception of each arrangement that includes variable consideration, the Company evaluates the amount of potential transaction price and the likelihood that the transaction price will be received. The Company utilizes either the most likely amount method or expected value method to estimate the amount expected to be received based on which method best predicts the amount expected to be received. The amount of variable consideration that is included in the transaction price may be constrained and is included in the transaction price only to the extent that it is probable that a significant reversal in the amount of the cumulative revenue recognized will not occur in a future period. Arrangements that include rights to additional goods or services that are exercisable at a customer’s discretion are generally considered options. We assess if these options provide a material right to the customer and, if so, these options are considered performance obligations. The Company has not currently identified any such material rights. Revenue is recognized when, or as, the Company satisfies a performance obligation by transferring a promised good or service to a customer. An asset is transferred when, or as, the customer obtains control of that asset. For performance obligations that are satisfied over time, the Company recognizes revenue using an input or output measure of progress that best depicts the satisfaction of the relevant performance obligation. After contract inception, the transaction price is reassessed at every period end and updated for changes such as resolution of uncertain events. Any change in the overall transaction price is allocated to the performance obligations on the same methodology as at contract inception. See note 11, License and Collaboration Agreements, for further discussion related to the Sobi collaboration agreement.
Offering Costs	Offering Costs Offering costs represent underwriting, legal, accounting and other direct costs related to the Company’s follow-on offerings and filing of a registration statements on Form S-3 in 2019, and related to the Company’s offering of Convertible Notes in 2020 and 2019. Costs were deferred until completion of the follow-on offerings and offering of Convertible Notes, at which time they were reclassified to additional paid-in capital as a reduction of the proceeds.
Segment Information	Segment Information Operating segments are defined as components of an enterprise about which separate discrete information is available for evaluation by the chief operating decision maker, or decision-making group, in deciding how to allocate resources and in assessing performance. The Company views its operations and manages its business in one operating segment.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results may differ from those estimates. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates, and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes, and management must select an amount that falls within that range of reasonable estimates. Estimates are used in the following areas, among others: development derivative liability, accrued expenses, prepaid expenses, convertible debt and taxes.
Fair Value of Financial Instruments	Fair Value of Financial Instruments The Company is required to disclose information on the fair value of financial instruments and inputs that enable an assessment of the fair value. The three levels of the fair value hierarchy prioritize valuation inputs based upon the observable nature of those inputs as follows: Level 1 – Quoted prices in active markets for identical assets or liabilities; Level 2 – Inputs other than quoted prices included within level 1 that are observable for the asset or liability, either directly or indirectly; Level 3 – Unobservable inputs that reflect the Company’s own assumptions about the assumptions market participants would use in pricing the asset or liability. The Company’s financial instruments, in addition to those presented in Note 6, Long-term Debt, Note 8, Marketable Securities, and Note 10, Fair Value Measurements, include cash and cash equivalents, the Australian research and development credit, accounts payable and accrued liabilities. Management believes that the carrying amounts of cash and cash equivalents, the Australian research and development credit, accounts payable and accrued expenses approximate the fair value due to the short-term nature of those instruments.
Cash and Cash Equivalents	Cash and Cash Equivalents Cash and cash equivalents are defined as cash in banks and investment instruments having maturities of three months or less from their acquisition date. The carrying amounts reported in the consolidated balance sheets for cash and cash equivalents are valued at cost, which approximates the fair value. See Note 10, Fair Value Measurements, for additional information.
Foreign Currency	Foreign Currency The financial position and results of operations of the Company's Australian, Irish and German subsidiaries are measured using the foreign subsidiary's local currency. Revenues and expenses of the subsidiaries have been translated into U.S. dollars at average exchange rates prevailing during the respective periods. Assets and liabilities have been translated at the rates of exchange on the balance sheet date. The resulting translation gain and loss adjustments are recorded directly as a separate component of stockholders’ equity. The financial position and results of operations of the Company’s Swiss subsidiary are measured and reported in U.S. dollars and transactions are translated to U.S. dollars at the end of the period.
Research and Development	Research and Development Costs incurred in connection with research and development activities are expensed as incurred. Research and development expenses include (i) employee-related expenses, including salaries, benefits, travel and share-based compensation expense; (ii) external research and development expenses incurred under arrangements with third parties, such as contract research and contract manufacturing organizations, investigational sites and consultants, including share-based compensation expense for consultants; (iii) the cost of acquiring, developing and manufacturing clinical study materials; and (iv) costs associated with preclinical and clinical activities and regulatory operations. The Company enters into consulting, research and other agreements with commercial entities, researchers, universities and others for the provision of goods and services. Such arrangements are generally cancellable upon reasonable notice and payment of costs incurred. Costs are considered incurred based on an evaluation of the progress to completion of specific tasks under each contract using information and data provided by the Company’s clinical sites and vendors. These costs consist of direct and indirect costs associated with specific projects, as well as fees paid to various entities that perform certain research on behalf of the Company. Depending upon the timing of payments to the service providers, the Company recognizes prepaid expenses or accrued expenses related to these costs. These accrued or prepaid expenses are based on management’s estimates of the work performed under service agreements, milestones achieved and experience with similar contracts. The Company monitors each of these factors and adjusts estimates accordingly. Included in research and development is a refundable Australian research and development credit. The credit is recognized as a reduction of clinical trial costs over the periods necessary to match the benefit of the credit with the costs for which it is intended to compensate.
Patents	Patents Costs incurred in connection with the application for and issuance of patents are expensed as incurred.
Income Taxes	Income Taxes The Company recognizes deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements or tax returns. Deferred tax assets and liabilities are determined based on the difference between the financial statement and tax bases of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Valuation allowances are provided if, based upon the weight of available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized. The Company accounts for uncertain tax positions in accordance with the provisions of ASC 740. When uncertain tax positions exist, the Company recognizes the tax benefit of tax positions to the extent that the benefit will more likely than not be realized. The determination as to whether the tax benefit will more likely than not be realized is based upon the technical merits of the tax position, as well as consideration of the available facts and circumstances. As of December 31, 2020 and 2019, the Company did not have any significant uncertain tax positions.
Concentrations of Credit Risk	Concentrations of Credit Risk Cash and cash equivalents are the only financial instruments that potentially subject the Company to concentrations of credit risk. Cash and cash equivalents are held at financial institutions in the United States, Switzerland, Australia, Ireland and Germany. The Company is exposed to credit risk in the event of default by the financial institution to the extent that cash and cash equivalent balances recorded in the balance sheets are in excess of the amounts that are insured by the Federal Deposit Insurance Corporation. The Company has not experienced any losses on its deposits since inception, and management believes that minimal credit risk exists with respect to these financial institutions.
Net Loss per Share	Net Loss per Share Basic net loss per common share is calculated by dividing net loss by the weighted-average shares outstanding during the period. For purposes of the diluted net loss per share calculation, convertible notes and common stock options are considered to be common stock equivalents but have been excluded from the calculation of diluted net loss per share, as their effect would be anti-dilutive for all periods presented. Therefore, basic and diluted net loss per share were the same for all periods presented.
Comprehensive Loss	Comprehensive Loss The Company’s components of comprehensive loss other than its net loss, are the foreign currency gains/losses recorded from the remeasurement of the long term intra-entity loan transaction to the Company’s wholly owned subsidiaries as well as the foreign currency gain/ loss from the translation of the Company’s wholly owned subsidiaries into U.S. dollars in 2020 and 2019.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In August 2020, the FASB issued ASU 2020-06, Debt – Debt with Conversion and Other Options (Subtopic 470-20) to reduce complexity in applying GAAP to certain financial instruments with characteristics of liability and equity. The ASU removes the guidance that requires entities to account for beneficial conversion features and cash conversion features in equity, separately from the host convertible debt or preferred stock. The ASU further revises the guidance to require entities to calculate diluted earnings per share for convertible instruments by using the if-converted method. In addition, entities must presume share settlement for purposes of calculating diluted EPS when an instrument may be settled in cash or shares. The new standard is effective for annual reporting periods beginning after December 15, 2021, for public companies, including interim periods within that reporting period. Early adoption is permitted, but no earlier than fiscal years beginning after December 15, 2020. The Company early adopted this statement effective January 1, 2021. The impact of the adoption of the statement is to increase debt and decrease equity by the amount of the equity component of convertible notes recognized in equity. Additionally, interest expense is expected to decrease by the non-cash portion of the discount amortization. Weighted average basic earnings per share amounts are not expected to be materially affected.

Common Stock (Tables)

12 Months Ended

Dec. 31, 2020

Summary of Shares of Common Stock Reserved for Future Issuance

The Company has reserved the following shares of common stock for future issuance (in thousands):

	December 31,
	2020		2019		2018
Shares reserved under 2010 Equity Incentive Plan		—		—		5,013
Shares reserved under 2017 Equity Incentive Plan		8,613		6,319		4,033
Shares reserved under 2017 Employee Stock Purchase Plan		913		972		972
Common stock warrants		—		—		14
Total		9,526		7,291		10,032

Development Derivative Liability (Tables)

12 Months Ended

Dec. 31, 2020

Research And Development [Abstract]

Schedule of Development Derivative Liability

The following table presents a rollforward of the liability (in thousands):

	For the Year Ended December 31,
	2020		2019
Balance at fair market value, January 1,	$	134,839	$	—
Amounts received under the SFJ agreement and SFJ amendment		20,000		120,000
Loss recorded in loss from remeasurement of development derivative liability		103,029		14,839
Balance at fair market value, December 31,	$	257,868	$	134,839

Accrued Expenses and Prepaid Assets (Tables)

12 Months Ended

Dec. 31, 2020

Accrued Expenses And Prepaid Assets [Abstract]

Schedule of Accrued Expenses

Accrued expenses are as follow (in thousands):

	December 31,
	2020		2019
Accrued research and development	$	47,879	$	36,449
Accrued license fee		25,050		—
Accrued payroll liabilities		22,896		11,443
Other		16,110		6,891
Total	$	111,935	$	54,783

Leases (Tables)

12 Months Ended

Dec. 31, 2020

Leases [Abstract]

Schedule of Additional Information Related to Operating Lease Assets and Liabilities

Additional information related to the operating lease assets and liabilities is as follows (in thousands):

	2020			2019
Operating lease assets	$	17,719		$	14,110
Operating lease liabilities	$	18,902		$	14,466
Weighted average remaining term in years		4.66		5.01
Weighted average discount rate used to measure outstanding lease liabilities		7.74	%		8.19	%

Supplemental Cash Flow Information Related to Operating Leases

Supplemental cash flow information related to operating leases for the years ended December 31, 2020 and 2019 is as follows (in thousands):

	2020		2019
Operating cash flows from operating leases	$	4,732	$	2,013
Operating lease assets obtained in exchange for lease obligations	$	7,237	$	10,201

Maturity of Operating Lease Liabilities

The maturity of the Company’s operating lease liabilities as of December 31, 2020 are as follows (in thousands):

2021	$	4,962
2022		4,842
2023		4,826
2024		4,114
2025 and thereafter		3,745
Total future minimum lease payments		22,489
Less imputed interest		(3,587	)
Total operating lease liabilities	$	18,902

Marketable Securities (Tables)

12 Months Ended

Dec. 31, 2020

Investments Debt And Equity Securities [Abstract]

Schedule of Amortized Cost, Gross Unrealized Holding Losses and Fair Value of Available-for-Sale Debt Securities by Type of Security

The amortized cost, gross unrealized holding losses and fair value of available-for-sale debt securities by type of security as of December 31, 2020 were as follows (in thousands):

	As of December 31, 2020
	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Fair Value
U.S. Government-related obligations	$	311,877	$	11	$	(19	)	$	311,869

Other Comprehensive Income and Accumulated Other Comprehensive Income (Tables)

12 Months Ended

Dec. 31, 2020

Summary of Changes in Accumulated Other Comprehensive Income/(Loss), by Component

The following tables summarize the changes in accumulated other comprehensive income/(loss), by component for the years ended December 31, 2020 and 2019 (in thousands):

	Unrealized Gains (Losses) from Marketable Securities			Foreign Currency Translation Adjustment			Total Accumulated Other Comprehensive Income (Loss)
Balances, December 31, 2019	$	—		$	(154	)	$	(154	)
Net other comprehensive income (loss)		(8	)		45			37
Balances, December 31, 2020	$	(8	)	$	(109	)	$	(117	)


	Unrealized Gains (Losses) from Marketable Securities			Foreign Currency Translation Adjustment			Total Accumulated Other Comprehensive Income (Loss)
Balances, December 31, 2018	$	—		$	(123	)	$	(123	)
Net other comprehensive income (loss)		—			(31	)		(31	)
Balances, December 31, 2019	$	—		$	(154	)	$	(154	)

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2020

Fair Value, Nonrecurring [Member]

Schedule of Financial Instruments and the Related Fair Value Hierarchy of the Valuation Techniques Utilized

The following table presents the fair value of financial instruments recorded originally at amortized cost or fair value and not re-measured on a recurring basis (in thousands):

		December 31, 2020
Balance Sheet Classification:	Type of Instrument	Level 1		Level 2		Level 3		Total
Financial Assets:
Cash and cash equivalents:	Money market funds	$	427,515	$	—	$	—	$	427,515
	Bank certificates of deposit		43,577		—		—		43,577
Total Financial Assets		$	471,092	$	—	$	—	$	471,092

		December 31, 2019
Balance Sheet Classification:	Type of Instrument	Level 1		Level 2		Level 3		Total
Financial Assets:
Cash and cash equivalents:	Money market funds	$	281,314	$	—	$	—	$	281,314
Total Financial Assets		$	281,314	$	—	$	—	$	281,314

Fair Value, Recurring [Member]

Schedule of Financial Instruments and the Related Fair Value Hierarchy of the Valuation Techniques Utilized

The following table presents the fair value of financial instruments recorded at fair value at inception and remeasured on a recurring basis (in thousands):

		December 31, 2020
Balance Sheet Classification:	Type of Instrument	Level 1		Level 2		Level 3		Total
Financial Assets:
Cash and cash equivalents:	US government obligations	$	89,990	$	—	$	—	$	89,990
Marketable securities:	US government obligations		311,869		—		—		311,869
Total Financial Assets		$	401,859	$	—	$	—	$	401,859

Financial Liabilities:
Development derivative liability	Development derivative liability	$	—	$	—	$	257,868	$	257,868
Total Financial Liabilities		$	—	$	—	$	257,868	$	257,868

		December 31, 2019
Balance Sheet Classification:	Type of Instrument	Level 1		Level 2		Level 3		Total
Financial Liabilities:
Development derivative liability	Development derivative liability	$	—	$	—	$	134,839	$	134,839
Total Financial Liabilities		$	—	$	—	$	134,839	$	134,839

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2020

Income Tax Disclosure [Abstract]

Components of Loss From Continuing Operation Before Provision for Income Taxes

The components of loss from continuing operations before provision for income taxes are as follows (in thousands):

	Year Ended December 31,
	2020			2019			2018
Domestic	$	(118,839	)	$	(297,127	)	$	(122,900	)
Foreign		(224,190	)		(7,580	)		(4,602	)
Total	$	(343,029	)	$	(304,707	)	$	(127,502	)

Schedule of Provision for Income Tax Expense

The provision for income taxes for the year ended December 31, 2020 is as follows (in thousands):

	Year Ended December 31, 2020
Current income tax expense:
U.S. Federal	$	—
U.S. State and Local		57
Foreign		1,788
Total current income tax expense		1,845
Deferred income tax expense:
U.S. Federal		—
U.S. State and Local		—
Foreign		—
Total deferred income tax expense		—
Total tax expense	$	1,845

Schedule of Effective Income Tax Rate Provision Amount Differs from Statutory U.S. Federal Income Tax Rate

The Company’s effective income tax provision differs from the amount calculated using the statutory U.S. federal income tax rate, principally due to the following (in thousands):

	Year Ended December
	2020						2019						2018
	Amount			Percentage of income before income taxes			Amount			Percentage of income before income taxes			Amount			Percentage of income before income taxes
Statutory U.S. federal income tax	$	(72,036	)		21.0	%	$	(63,988	)		21.0	%	$	(26,775	)		21.0	%
Foreign tax rate differential		22,760			(6.6	)		—			—			—			—
State income taxes, net of federal benefit		(14,107	)		4.1			(16,424	)		5.4			(6,398	)		5.0
Change in valuation allowances		240,065			(69.9	)		90,300			(29.6	)		38,157			(29.9	)
Intellectual property transfer		(162,000	)		47.2			—			—			—			—
Tax credits		(11,696	)		3.4			(6,113	)		2.0			(6,149	)		4.8
Change in state apportionment, Kentucky tax reform and other		93			—			(17	)		—			873			(0.7	)
Permanent and other including share based payments excess deductions		(1,234	)		0.3			(3,758	)		1.2			292			(0.2	)
Effective income tax provision	$	1,845			(0.5	)	$	—			—	%	$	—			—	%

Schedule of Deferred Tax Assets and Liabilities

The following table presents the principal components of the Company’s deferred tax assets and liabilities (in thousands):

	December 31,
	2020			2019
Deferred tax assets:
Intangible assets	$	162,011		$	16
Share-based compensation		11,827			6,625
Deferred interest expense		—			—
Contribution carryforwards		—			41
Net operating loss carryforwards		122,731			102,623
Research and development credits		24,992			14,476
Orphan drug credits		14,372			9,380
Development derivative liability		68,883			35,875
Accruals		4,520			2,259
Other		207			11
Total deferred tax assets		409,543			171,306
Deferred tax liabilities:
Fixed assets		(23	)		—
Convertible debt		(10,473	)		(11,480	)
481(a) adjustment		(1,935	)		(2,891	)
Total deferred tax liabilities		(12,431	)		(14,371	)
Net deferred tax assets before allowance:		397,112			156,935
Less valuation allowance		(397,112	)		(156,935	)
Net deferred tax assets	$	—		$	—

Equity Incentive Plans (Tables)

12 Months Ended

Dec. 31, 2020

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Share-based Compensation Expense

Total share-based compensation expense related to the various plans during the years ended was as follows (in thousands):

	Year Ended December 31,
	2020		2019		2018
Research and development	$	21,381	$	10,683	$	3,559
General and administrative		23,995		10,461		4,174
Total share-based compensation expense	$	45,376	$	21,144	$	7,733

Summary of Stock Option Activity

The following table summarizes the Company’s stock option activity:

				Weighted -		Weighted -
				Average		Average		Aggregate
				Exercise		Contractual		Intrinsic
	Shares			Price		Life		Value
	(in thousands)			Per Share		(in years)		(in thousands)
Outstanding, December 31, 2019		10,855		$	13.52
Granted		2,884			37.93
Exercised		(1,208	)		7.80
Forfeited		(795	)		28.03
Outstanding, December 31, 2020		11,736		$	19.13
Options exercisable, December 31, 2020		6,062		$	10.96		6.22	$	280,290
Expected to vest, December 31, 2020		5,674		$	27.85		8.62	$	166,527

Summary of Assumption Used to Estimate the Grant Date Fair Value

The assumptions used in the Black-Scholes model to estimate the grant date fair value are as follows:

	Year Ended December 31,
	2020	2019	2018
Risk-free interest rate	0.32 - 1.76%	1.42 - 2.56%	2.68 - 2.99%
Dividend yield	0%	0%	0%
Volatility	84.4 - 87.8%	102.6 - 111.2%	87.0 - 110.9%
Expected terms (years)	5.31 - 6.08	5.31 - 6.08	5.94 - 6.25

Schedule of Unvested Restricted Stock Units Activity

	Number of Stock Units (in thousands)			Weighted Average Grant Date Fair Value Per Share
Unvested Balance at December 31, 2019		—		$	—
Granted		546			38.97
Vested		—			—
Forfeited		(44	)		40.43
Unvested Balance at December 31, 2020		502			38.84

Net Loss per Common Share (Tables)

12 Months Ended

Dec. 31, 2020

Earnings Per Share [Abstract]

Summary of Basic and Diluted Net Loss per Common Share

The following table presents the calculation of basic and diluted net loss per common share (amounts in thousands except per share amounts):

	Year Ended December 31,
	2020			2019			2018
Numerator:
Net loss	$	(344,874	)	$	(304,707	)	$	(127,502	)
Denominator:
Weighted-average number of common shares used in net loss per common share -- basic and diluted		75,163			62,229			54,396
Net loss per common share -- basic and diluted	$	(4.59	)	$	(4.90	)	$	(2.34	)

Summary of Shares Outstanding that were Excluded from Calculation of Diluted Net Loss Per Share

Shares outstanding presented below were excluded from the calculation of diluted net loss per share, prior to the use of the treasury stock method, as their effect is anti-dilutive (in thousands):

	Year Ended December 31,
	2020		2019		2018
Convertible notes		13,177		—		—
Common stock under option		11,736		10,854		7,498
Restricted stock units		502		—		—
Common stock warrants		—		—		14
Total		25,415		10,854		7,512

Selected Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2020

Quarterly Financial Information Disclosure [Abstract]

Summary of Interim Financial Information on Quarterly Basis

The following interim financial information presents the Company’s 2020 and 2019 results of operations on a quarterly basis (amounts in thousands except per share amounts):

	Quarter Ended
	March 31,			June 30,			September 30,			December 31,
	2020			2020			2020			2020
Operating income/ (loss)	$	(98,786	)	$	(115,508	)	$	(129,552	)	$	130,120
Net income/(loss)		(168,822	)		(118,617	)		(135,700	)		78,265
Net income/(loss) per common share, basic (1)		(2.29	)		(1.57	)		(1.79	)		1.03
Net income/(loss) per common share, diluted (1)		(2.29	)		(1.57	)		(1.79	)		0.93

	Quarter Ended
	March 31,			June 30,			September 30,			December 31,
	2019			2019			2019			2019
Operating Loss	$	(48,651	)	$	(63,476	)	$	(69,948	)	$	(105,940	)
Net Loss		(50,574	)		(71,090	)		(69,825	)		(113,218	)
Net Loss per common share, basic and diluted (1)		(0.87	)		(1.12	)		(1.10	)		(1.77	)

(1)

Nature of Organization and Operations - Additional Information (Detail) $ / shares in Units, $ in Thousands								1 Months Ended	12 Months Ended
	Jan. 06, 2021 USD ($) shares	Nov. 30, 2020 USD ($)	Oct. 27, 2020 USD ($)	May 12, 2020 USD ($)	Jan. 13, 2020 USD ($) $ / shares shares	Sep. 16, 2019 USD ($)	Mar. 11, 2019 USD ($) $ / shares shares	Nov. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($) shares	May 06, 2020 CounterParty $ / shares	Sep. 11, 2019 CounterParty $ / shares
Nature Of Organization And Operations [Line Items]
Proceeds from follow-on public offering after deducting underwriting discounts and commissions									$ 381,423	$ 109,581	$ 131,194
Cash and cash equivalents and marketable securities									$ 877,600
Common Stock [Member]
Nature Of Organization And Operations [Line Items]
Issuance of common stock in follow-on offering, shares \| shares									10,925	6,900	5,500
Capped Call Transactions [Member]
Nature Of Organization And Operations [Line Items]
Number of counterparties \| CounterParty												2	2
Initial conversion price \| $ / shares												$ 39.4625	$ 39.4625
Follow-on Public Offerings [Member]
Nature Of Organization And Operations [Line Items]
Issuance of common stock in follow-on offering, shares \| shares					10,925,000		6,900,000
Price of common stock \| $ / shares					$ 37.00		$ 17.00
Proceeds from follow-on public offering after deducting underwriting discounts and commissions					$ 381,400		$ 109,600
Underwriting discounts and commissions					22,200		7,000
Offering Cost					$ 500		$ 700
Over Allotment Option [Member]
Nature Of Organization And Operations [Line Items]
Issuance of common stock in follow-on offering, shares \| shares					1,425,000
Price of common stock \| $ / shares					$ 37.00
Convertible Senior Notes Due 2026 [Member]
Nature Of Organization And Operations [Line Items]
Debt instrument, stated percentage				3.50%
Debt instrument, due and payment description									The 2020 Convertible Notes are senior unsecured obligations of the Company and bear interest at a rate of 3.5% per year payable semiannually in arrears on March 15 and September 15 of each year, beginning on March 15, 2020.
Debt instrument, frequency of periodic payment									semiannually
Maturity date				Sep. 15, 2026
Convertible Senior Notes Due 2026 [Member] \| 2020 Convertible Notes [Member]
Nature Of Organization And Operations [Line Items]
Debt instrument, stated percentage				3.50%
Debt instrument maturity year				2026
Purchase price of convertible notes				$ 328,900
Net proceeds from the sale of the notes				322,900
Payments of transactions cost				43,100
Convertible Senior Notes Due 2026 [Member] \| Private Offering [Member] \| 2020 Convertible Notes [Member]
Nature Of Organization And Operations [Line Items]
Principal amount				300,000
Convertible Senior Notes Due 2026 [Member]
Nature Of Organization And Operations [Line Items]
Principal amount									$ 520,000
Debt instrument, stated percentage						3.50%
Debt instrument, due and payment description									The Convertible Notes are senior unsecured obligations of the Company and bear interest at a rate of 3.5% per year payable semiannually in arrears on March 15 and September 15 of each year, beginning on March 15, 2020.
Debt instrument, frequency of periodic payment									semiannually
Maturity date						Sep. 15, 2026
Convertible Senior Notes Due 2026 [Member] \| 2020 Convertible Notes [Member]
Nature Of Organization And Operations [Line Items]
Net proceeds from the sale of the notes				322,900
Payments of transactions cost				43,100
Convertible Senior Notes Due 2026 [Member] \| 2019 Convertible Notes [Member]
Nature Of Organization And Operations [Line Items]
Debt instrument, stated percentage						3.50%
Net proceeds from the sale of the notes						$ 212,900
Payments of transactions cost						28,400
Convertible Senior Notes Due 2026 [Member] \| Private Offering [Member] \| 2020 Convertible Notes [Member]
Nature Of Organization And Operations [Line Items]
Principal amount				$ 300,000
Convertible Senior Notes Due 2026 [Member] \| Private Offering [Member] \| 2019 Convertible Notes [Member]
Nature Of Organization And Operations [Line Items]
Principal amount						$ 220,000
Swedish Orphan Biovitrum AB (Publ) [Member]
Nature Of Organization And Operations [Line Items]
Development cost reimbursement			$ 80,000
Collaboration and License Agreement [Member]
Nature Of Organization And Operations [Line Items]
Development cost reimbursement									$ 80,000
Collaboration and License Agreement [Member] \| Swedish Orphan Biovitrum AB (Publ) [Member]
Nature Of Organization And Operations [Line Items]
Upfront Payment		$ 250,000						$ 250,000
Development cost reimbursement			$ 80,000						80,000
Royalties entitled to receive after first commercial sale of applicable licensed product period			10 years
Collaboration and License Agreement [Member] \| Swedish Orphan Biovitrum AB (Publ) [Member] \| Maximum [Member]
Nature Of Organization And Operations [Line Items]
Aggregate milestone payments upon achievement of specified one-time regulatory and commercial milestone events			$ 915,000
Exchange Agreements [Member] \| Convertible Senior Notes Due 2026 [Member] \| 2019 Convertible Notes [Member]
Nature Of Organization And Operations [Line Items]
Aggregate principal amount converted									$ 126,100
Exchange Agreements [Member] \| Convertible Senior Notes Due 2026 [Member] \| 2019 Convertible Notes [Member] \| Subsequent Event [Member]
Nature Of Organization And Operations [Line Items]
Aggregate principal amount converted	$ 126,100
Exchange Agreements [Member] \| Convertible Senior Notes Due 2026 [Member] \| 2019 Convertible Notes [Member] \| Subsequent Event [Member] \| Common Stock [Member]
Nature Of Organization And Operations [Line Items]
Aggregate principal amount converted into shares \| shares	3,906,869

Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2020 Segment
Schedule Of Summary Of Significant Accounting Policies [Line Items]
Number of operating segment	1
ASU 2020-06 [Member]
Schedule Of Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle accounting standards update adopted	true
Change in accounting principle accounting standards update adoption date	Jan. 01, 2021

Indicates (true false) whether accounting standards update was adopted.

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Date accounting standards update was adopted, in CCYY-MM-DD format.

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Common Stock - Summary of Shares of Common Stock Reserved for Future Issuance (Detail) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2020	Dec. 31, 2019
Class Of Stock [Line Items]
Common stock reserved for future issuance	10,032,000	9,526,000	7,291,000
Common stock warrants	14,000
2010 Stock Incentive Plan [Member]
Class Of Stock [Line Items]
Common stock reserved for future issuance	5,013,000
2017 Stock Incentive Plan [Member]
Class Of Stock [Line Items]
Common stock reserved for future issuance	4,033,000	8,613,049	6,319,000
2017 Employee Stock Purchase Plan [Member]
Class Of Stock [Line Items]
Common stock reserved for future issuance	972,000	913,000	972,000

Development Derivative Liability - Additional Information (Detail)		12 Months Ended
	Feb. 28, 2019 USD ($) Installment	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jun. 07, 2019 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Loss from remeasurement of development derivative liability		$ 103,029,000	$ 14,839,000
Development derivative liability		253,638,000	134,839,000
Current portion of development derivative liability		$ 4,230,000
Level 3 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Implied cost of borrowing discount rates		12.65%
SFJ Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Proceeds from SFJ agreement	$ 60,000,000.0	$ 20,000,000.0	120,000,000.0
Additional funding amount upon achievement of development milestones	$ 60,000,000.0
Number of milestone payments \| Installment	3
Development funding for minimum period of operating expense	10 months
Increase in additional funding for development costs				$ 20,000,000.0
Number of additional annual payments \| Installment	6
Loss from remeasurement of development derivative liability		103,000,000.0	14,800,000
Proceeds from collaborators of additional milestones		20,000,000.0	60,000,000.0
Development derivative liability		257,900,000	$ 134,800,000
Current portion of development derivative liability		$ 4,200,000
SFJ Agreement [Member] \| Level 3 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Implied cost of borrowing discount rates		8.00%
SFJ Agreement [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Obligated to pay initial payment	$ 5,000,000.0
Aggregate amount of additional annual payments	226,000,000.0
SFJ Agreement [Member] \| Regulatory Approval Granted by FDA and EMA [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Obligated to pay initial payment	10,000,000.0
SFJ Agreement [Member] \| Regulatory Approval Granted by FDA and EMA [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate amount of additional annual payments	$ 452,000,000.0

Development Derivative Liability - Schedule of Development Derivative Liability (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Balance at fair market value	$ 134,839
Amounts received under the SFJ agreement and SFJ amendment	20,000	$ 120,000
Loss recorded in loss from remeasurement of development derivative liability	103,029	14,839
Balance at fair market value	$ 257,868	$ 134,839

Accrued Expenses and Prepaid Assets - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Accrued Expenses And Prepaid Assets [Abstract]
Accrued research and development	$ 47,879	$ 36,449
Accrued license fee	25,050
Accrued payroll liabilities	22,896	11,443
Other	16,110	6,891
Total	$ 111,935	$ 54,783

Accrued Expenses and Prepaid Assets - Additional Information (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Accrued Expenses And Prepaid Assets [Abstract]
Prepaid research and development	$ 8.0	$ 18.1

Long-term Debt - Additional Information (Detail) $ / shares in Units, $ in Thousands							4 Months Ended	12 Months Ended
	Jan. 06, 2021 USD ($)	May 12, 2020 USD ($)	Sep. 16, 2019 USD ($)	Mar. 26, 2019 USD ($)	Oct. 20, 2017 USD ($) Installment $ / shares shares	Oct. 19, 2017 USD ($) $ / shares shares	Dec. 31, 2019 USD ($)	Dec. 31, 2020 USD ($) d $ / shares	Dec. 31, 2019 USD ($)	Dec. 31, 2018 shares	May 06, 2020 CounterParty $ / shares	Sep. 11, 2019 USD ($) CounterParty $ / shares
Debt Instrument [Line Items]
Amortization of debt discounts									$ 52
Loss on extinguishment of debt									1,501
Warrant to purchase common stock \| shares										14,000
Term Loan Facility [Member]
Debt Instrument [Line Items]
Principal amount				$ 21,700
Final payment due amount				21,800
Debt instrument per diem interest amount				$ 100
Final payment percentage equal to original principal amount				8.00%
Final payment towards original principal amount				$ 1,600
Prepayment fee				100
Loss on extinguishment of debt				$ 1,200
Silicon Valley Bank [Member] \| Term Loan Facility [Member]
Debt Instrument [Line Items]
Warrant to purchase common stock \| shares					14,064
Exercise price per share \| $ / shares					$ 5.484
Warrant term					10 years
Aggregate purchase price					$ 250
Loan and Security Agreement [Member] \| Silicon Valley Bank [Member]
Debt Instrument [Line Items]
Debt instrument, due and payment description								the Company was required to make monthly interest-only payments through November 1, 2019 and to make 24 equal monthly payments of principal, plus accrued interest, thereafter from November 1, 2019 through October 1, 2021, when all unpaid principal and interest would become due and payable
Term loan facility					$ 20,000
Loan and Security Agreement [Member] \| Silicon Valley Bank [Member] \| Convertible Preferred Stock [Member]
Debt Instrument [Line Items]
Debt instrument, number of equal monthly installments \| Installment					24
Loan and Security Agreement [Member] \| Silicon Valley Bank [Member] \| Prime Rate [Member]
Debt Instrument [Line Items]
Interest bear rate					1.50%
2019 Convertible Notes [Member]
Debt Instrument [Line Items]
Premium paid for capped call transactions								$ 28,400
2020 Convertible Notes [Member]
Debt Instrument [Line Items]
Premium paid for capped call transactions								43,100
Capped Call Transactions [Member]
Debt Instrument [Line Items]
Number of counterparties \| CounterParty											2	2
Initial conversion price \| $ / shares											$ 39.4625	$ 39.4625
Initial cap price \| $ / shares												63.14
Sale price \| $ / shares												$ 31.57
Convertible Senior Notes Due 2026 [Member]
Debt Instrument [Line Items]
Principal amount								$ 520,000
Debt instrument, stated percentage			3.50%
Debt instrument, due and payment description								The Convertible Notes are senior unsecured obligations of the Company and bear interest at a rate of 3.5% per year payable semiannually in arrears on March 15 and September 15 of each year, beginning on March 15, 2020.
Debt instrument, frequency of periodic payment								semiannually
Maturity date			Sep. 15, 2026
Redemption period, start date								Sep. 20, 2023
Threshold percentage of stock price trigger								130.00%
Threshold trading days \| d								20
Threshold consecutive trading days \| d								30
Redemption price, percentage								100.00%
Debt interest expense							$ 4,400	$ 29,900
Amortization of debt discounts							2,100	15,000
Accrued semi annual coupon payable							2,200	14,400
Amortization of debt issuance costs							100	500
Debt issuance costs gross							4,600	7,800	4,600
Long-term debt							$ 142,600	$ 358,800	$ 142,600
Convertible Senior Notes Due 2026 [Member] \| Common Stock [Member]
Debt Instrument [Line Items]
Terms of conversion								The Convertible Notes are convertible into shares of the Company’s common stock at an initial conversion rate of 25.3405 shares per $1,000 principal amount of Convertible Notes (equivalent to an initial conversion price of approximately $39.4625 per share of common stock).
Conversion ratio								0.0253405
Conversion price \| $ / shares								$ 39.4625
Convertible Senior Notes Due 2026 [Member] \| 2019 Convertible Notes [Member]
Debt Instrument [Line Items]
Net proceeds from the sale of the notes			$ 212,900
Payment of convertible debt discounts and commissions			6,600
Payment of convertible debt offering expenses			500
Payments of transactions cost			$ 28,400
Debt instrument, stated percentage			3.50%
Debt instrument effective interest rate												10.50%
Debt instrument convertible carrying amount of liability component												$ 145,100
Debt discount												74,900
Debt issuance costs												7,100
Convertible Senior Notes Due 2026 [Member] \| 2019 Convertible Notes [Member] \| Exchange Agreements [Member]
Debt Instrument [Line Items]
Aggregate principal amount converted								$ 126,100
Convertible Senior Notes Due 2026 [Member] \| 2019 Convertible Notes [Member] \| Subsequent Event [Member] \| Exchange Agreements [Member]
Debt Instrument [Line Items]
Aggregate principal amount converted	$ 126,100
Convertible Senior Notes Due 2026 [Member] \| 2019 Convertible Notes [Member] \| Liability Component [Member]
Debt Instrument [Line Items]
Debt issuance costs												4,700
Convertible Senior Notes Due 2026 [Member] \| 2019 Convertible Notes [Member] \| Equity Component [Member]
Debt Instrument [Line Items]
Debt issuance costs												$ 2,400
Convertible Senior Notes Due 2026 [Member] \| 2020 Convertible Notes [Member]
Debt Instrument [Line Items]
Net proceeds from the sale of the notes		$ 322,900
Payment of convertible debt discounts and commissions		5,700
Payment of convertible debt offering expenses		300
Payments of transactions cost		$ 43,100
Debt instrument effective interest rate		10.50%
Debt instrument convertible carrying amount of liability component		$ 204,500
Debt discount		95,500
Debt issuance costs		6,000
Convertible Senior Notes Due 2026 [Member] \| 2020 Convertible Notes [Member] \| Liability Component [Member]
Debt Instrument [Line Items]
Debt issuance costs		3,700
Convertible Senior Notes Due 2026 [Member] \| 2020 Convertible Notes [Member] \| Equity Component [Member]
Debt Instrument [Line Items]
Debt issuance costs		2,300
Convertible Senior Notes Due 2026 [Member] \| Private Offering [Member] \| 2019 Convertible Notes [Member]
Debt Instrument [Line Items]
Principal amount			$ 220,000
Convertible Senior Notes Due 2026 [Member] \| Private Offering [Member] \| 2020 Convertible Notes [Member]
Debt Instrument [Line Items]
Principal amount		$ 300,000
Unsecured Promissory Note [Member] \| Golda Darty Partners S.A. [Member]
Debt Instrument [Line Items]
Principal amount			7,000			$ 7,000
Debt instrument, due and payment description								The promissory note accrued interest at a rate per annum of 8.0%, and was due and payable quarterly in arrears on the 19th day of each April, July, October and January.
Maturity date						Oct. 19, 2022
Long-term debt						$ 7,000
Debt instrument per diem interest amount			100
Loss on extinguishment of debt						$ 300
Warrant to purchase common stock \| shares						93,764
Exercise price per share \| $ / shares						$ 5.484
Accrued interest rate						8.00%
Repayment of related party due amount including diem interest			$ 7,100
Fair value of warrant						$ 400

Leases - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Jan. 01, 2019
Lessee Lease Description [Line Items]
Operating lease asset	$ 17,719	$ 14,110
Operating lease liability	$ 18,902	14,466
Operating lease description	The Company enters into lease agreements with terms generally ranging from 2-7 years. Some of the Company’s lease agreements include Company options to extend the lease on a month-to-month basis or for set periods for up to five years. Many leases also include options to terminate the leases within one year or per other contractual terms. Renewal and termination options were generally not included in the lease term for the Company’s existing operating leases.
Operating lease, existence of option to extend	true
Operating lease maximum term of options to terminate lease	1 year
Operating lease expense	$ 4,400	2,400
Minimum [Member]
Lessee Lease Description [Line Items]
Operating lease, term of contract	2 years
Maximum [Member]
Lessee Lease Description [Line Items]
Operating lease, term of contract	7 years
Operating lease options to extend lease	5 years
ASU 2016-02 [Member]
Lessee Lease Description [Line Items]
Operating lease asset			$ 5,500
Operating lease liability			$ 5,500
Additional operating lease asset		9,800
Additional operating lease liability		$ 9,800

Leases - Schedule of Additional Information Related to Operating Lease Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating lease assets	$ 17,719	$ 14,110
Operating lease liabilities	$ 18,902	$ 14,466
Weighted average remaining term in years	4 years 7 months 28 days	5 years 3 days
Weighted average discount rate used to measure outstanding lease liabilities	7.74%	8.19%

Leases - Supplemental Cash Flow Information Related to Operating Leases (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating cash flows from operating leases	$ 4,732	$ 2,013
Operating lease assets obtained in exchange for lease obligations	$ 7,237	$ 10,201

Leases - Maturity of Operating Lease Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
2021	$ 4,962
2022	4,842
2023	4,826
2024	4,114
2025 and thereafter	3,745
Total future minimum lease payments	22,489
Less imputed interest	(3,587)
Operating lease liabilities	$ 18,902	$ 14,466

Marketable Securities - Schedule of Amortized Cost, Gross Unrealized Holding Losses and Fair Value of Available-for-Sale Debt Securities by Type of Security (Detail) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Available for sale securities:
Fair Value		$ 0
U.S. Government-Related Obligations [Member]
Available for sale securities:
Amortized Cost	$ 311,877,000
Gross Unrealized Holding Gains	11,000
Gross Unrealized Holding Losses	(19,000)
Fair Value	$ 311,869,000

Marketable Securities - Additional Information (Detail)	Dec. 31, 2019 USD ($)
Investments Debt And Equity Securities [Abstract]
Available-for-sale securities	$ 0

Other Comprehensive Income and Accumulated Other Comprehensive Income - Summary of Changes in Accumulated Other Comprehensive Income/(Loss), by Component (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning balance	$ 34,229	$ 160,973
Net other comprehensive income (loss)	37	(31)
Ending balance	204,557	34,229
Unrealized Gains (Losses) from Marketable Securities [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Net other comprehensive income (loss)	(8)
Ending balance	(8)
Foreign Currency Translation Adjustment [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning balance	(154)	(123)
Net other comprehensive income (loss)	45	(31)
Ending balance	(109)	(154)
Total Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning balance	(154)	(123)
Ending balance	$ (117)	$ (154)

Amount after tax and reclassification adjustments of other comprehensive income (loss).

Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.

Fair Value Measurements - Schedule of Financial Instruments and the Related Fair Value Hierarchy of the Valuation Techniques Utilized (Detail) - Fair Value, Nonrecurring [Member] - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Assets	$ 471,092	$ 281,314
Bank Certificates of Deposit [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Assets	43,577
Money Market Funds [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Assets	427,515	281,314
Level 1 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Assets	471,092	281,314
Level 1 [Member] \| Bank Certificates of Deposit [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Assets	43,577
Level 1 [Member] \| Money Market Funds [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Assets	$ 427,515	$ 281,314

Fair Value Measurements - Additional Information (Detail) - Level 3 [Member] - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Percentage of cost of borrowing	12.65%
Derivative liability measurement input	0.10
SFJ Agreement [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Percentage of cost of borrowing	8.00%
Convertible Senior Notes [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Convertible notes	$ 676,200,000	$ 149,500,000
Convertible notes, interest rate	3.50%	3.50%
Development Derivative Liability [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Change in fair value of derivative liability, percentage	5.00%
Change in fair value of derivative liability	$ 12,900

Fair Value Measurements - Schedule of Financial Instruments and the Related Fair Value Hierarchy of the Valuation Techniques Utilized (Details) - Fair Value, Recurring [Member] - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Assets	$ 401,859
Total Financial Liabilities	257,868	$ 134,839
Cash and Cash Equivalents [Member] \| US Government Obligations [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Assets	89,990
Marketable Securities [Member] \| US Government Obligations [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Assets	311,869
Development Derivative Liability [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Liabilities	257,868	134,839
Level 1 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Assets	401,859
Level 1 [Member] \| Cash and Cash Equivalents [Member] \| US Government Obligations [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Assets	89,990
Level 1 [Member] \| Marketable Securities [Member] \| US Government Obligations [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Assets	311,869
Level 3 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Liabilities	257,868	134,839
Level 3 [Member] \| Development Derivative Liability [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Liabilities	$ 257,868	$ 134,839

License and Collaboration Agreements - Additional Information (Detail) - USD ($) $ in Thousands			1 Months Ended	12 Months Ended
	Nov. 30, 2020	Oct. 27, 2020	Nov. 30, 2020	Dec. 31, 2020
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue				$ 250,646
Contra research and development expense				43,000
Contra research and development current				25,000
Contra research and development long-term assets				18,000
License [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue				250,494
Swedish Orphan Biovitrum AB (Publ) [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development cost reimbursement		$ 80,000
Non-refundable Upfront Payment		250,000
Revenue				600
Collaboration and License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development cost reimbursement				80,000
Collaboration and License Agreement [Member] \| License [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue				250,000
Collaboration and License Agreement [Member] \| Swedish Orphan Biovitrum AB (Publ) [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront Payment	$ 250,000		$ 250,000
Development cost reimbursement		$ 80,000		80,000
Royalties entitled to receive after first commercial sale of applicable licensed product period		10 years
Collaboration and License Agreement [Member] \| Swedish Orphan Biovitrum AB (Publ) [Member] \| License [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue				$ 250,000
Collaboration and License Agreement [Member] \| Swedish Orphan Biovitrum AB (Publ) [Member] \| ASC Topic 808
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront Payment	$ 250,000
Development cost reimbursement		$ 80,000
Collaboration and License Agreement [Member] \| Swedish Orphan Biovitrum AB (Publ) [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate milestone payments upon achievement of specified one-time regulatory and commercial milestone events		915,000
Collaboration and License Agreement [Member] \| Swedish Orphan Biovitrum AB (Publ) [Member] \| Maximum [Member] \| ASC Topic 808
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate milestone payments upon achievement of specified one-time regulatory and commercial milestone events		$ 915,000

License Agreements - Additional Information (Detail)	1 Months Ended	12 Months Ended
License Agreements - Additional Information (Detail)	Sep. 30, 2014 USD ($)	Dec. 31, 2020 USD ($) License	Dec. 31, 2018 USD ($)
Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future milestone payments		$ 15,000,000.0
Payment of royalty fees		6.00%
Minimum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Payment of royalty fees		3.00%
University of Pennsylvania [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Annual maintenance fees	$ 100,000
Annual milestone payment made			$ 1,000,000.0
University of Pennsylvania [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate milestones payments	3,200,000
Milestone payments based on annual sales milestones	$ 5,000,000.0
2010 License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Annual maintenance fees		$ 100,000
Number of licensed products \| License		2
2010 License Agreement [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate milestones payments		$ 1,700,000
Milestone payments based on annual sales milestones		2,500,000
Sobi Agreement and Another Licensing Transaction [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Royalty expense owed		$ 25,000,000.0

401(k) Profit Sharing Plan and Trust - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Compensation And Retirement Disclosure [Abstract]
Defined Contribution Plan, Plan Name [Extensible List]	apls:FourZeroOneKRetirementSavingsPlanMember	apls:FourZeroOneKRetirementSavingsPlanMember	apls:FourZeroOneKRetirementSavingsPlanMember
Minimum age of full-time employees eligible to participate in the plan	21 years
Employer contributions made to the 401(k) Plan	$ 2.1	$ 0.8	$ 0.3

Income Taxes - Components of Loss From Continuing Operation Before Provision for Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Components Of Income Tax Expense Benefit Continuing Operations [Abstract]
Domestic	$ (118,839)	$ (297,127)	$ (122,900)
Foreign	(224,190)	(7,580)	(4,602)
Total	$ (343,029)	$ (304,707)	$ (127,502)

Income Taxes - Components of Provision for Income Taxes (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Current income tax expense:
U.S. State and Local	$ 57,000
Foreign	1,788,000
Total current income tax expense	1,845,000	$ 0	$ 0
Deferred income tax expense:
Total deferred income tax expense		$ 0	$ 0
Total tax expense	$ 1,845,000

Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

Weighted-average number of common shares used in net loss per common share -- basic and diluted

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Taxes [Line Items]
Current income tax expense or benefit	$ 1,845,000	$ 0	$ 0
Deferred income tax expense or benefit		0	$ 0
Net impact on effective tax rate	0
Deferred tax asset, increase in valuation allowance	$ 240,200,000
Net operating loss carryforward period	3 years
Cumulative changes in ownership interest of significant stockholders	50.00%
Interest and penalties	$ 0	0
Accrued for interest and penalties	$ 0	0
Income tax examination description	Generally, the tax years 2017 through 2019 remain open and subject to examination by the major taxing jurisdictions to which the Company is subject.
Federal [Member]
Income Taxes [Line Items]
Net operating loss carryforwards	$ 358,200,000	392,900,000
Research and development tax credit carryforwards	34,200,000
Net operating loss carryforward indefinitely	$ 276,900,000
Net operating loss carryforward expiration year	2025
Research and development tax credit carryforward expiration year	2025
State [Member]
Income Taxes [Line Items]
Net operating loss carryforwards	$ 405,000,000.0	$ 358,300,000
Research and development tax credit carryforwards	$ 6,500,000
Net operating loss carryforward expiration year	2025
Research and development tax credit carryforward expiration year	2025
Foreign [Member]
Income Taxes [Line Items]
Net operating loss carryforwards	$ 240,900,000
Net operating loss carryforward expiration year	2026

Income Taxes - Schedule of Effective Income Tax Rate Provision Amount Differs from Statutory U.S. Federal Income Tax Rate (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Expense Benefit Continuing Operations [Abstract]
Statutory U.S. federal income tax, Amount	$ (72,036)	$ (63,988)	$ (26,775)
Foreign tax rate differential	22,760
State income taxes, net of federal benefit, Amount	(14,107)	(16,424)	(6,398)
Change in valuation allowances, Amount	240,065	90,300	38,157
Intellectual property transfer	(162,000)
Tax credits, Amount	(11,696)	(6,113)	(6,149)
Change in state apportionment, Kentucky tax reform and other, Amount	93	(17)	873
Permanent and other including share based payments excess deductions	(1,234)	$ (3,758)	$ 292
Total tax expense	$ 1,845

Income Taxes - Schedule of Effective Income Tax Rate Provision Differs Amount from Statutory U.S. Federal Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Expense Benefit Continuing Operations [Abstract]
Statutory U.S. federal income tax, Percentage of income before income taxes	21.00%	21.00%	21.00%
Foreign tax rate differential	(6.60%)
State income taxes, net of federal benefit, Percentage of income before income taxes	4.10%	5.40%	5.00%
Change in valuation allowances, Percentage of income before income taxes	(69.90%)	(29.60%)	(29.90%)
Intellectual property transfer	47.20%
Tax credits, Percentage of income before income taxes	3.40%	2.00%	4.80%
Change in state apportionment, Kentucky tax reform and other, Percentage of income before income taxes			(0.70%)
Permanent and other including share based payments excess deductions	0.30%	1.20%	(0.20%)
Effective income tax provision, Percentage of income before income taxes	(0.50%)

Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Intangible assets	$ 162,011	$ 16
Share-based compensation	11,827	6,625
Contribution carryforwards		41
Net operating loss carryforwards	122,731	102,623
Research and development credits	24,992	14,476
Orphan drug credits	14,372	9,380
Development derivative liability	68,883	35,875
Accruals	4,520	2,259
Other	207	11
Total deferred tax assets	409,543	171,306
Deferred tax liabilities:
Fixed assets	(23)
Convertible debt	(10,473)	(11,480)
481(a) adjustment	(1,935)	(2,891)
Total deferred tax liabilities	(12,431)	(14,371)
Net deferred tax assets before allowance:	397,112	156,935
Less valuation allowance	$ (397,112)	$ (156,935)

Commitments and Contingencies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2020 USD ($) ft²
Commitments And Contingencies [Line Items]
Base rent payable	$ 22,489,000
Incurred any cost to defend lawsuits or settle claims	0
Estimated termination costs and other fees	2,800,000
Non-cancellable purchase commitments	$ 13,700,000
Waltham, Massachusetts [Member] \| Amended Waltham Office Space Lease [Member]
Commitments And Contingencies [Line Items]
Operating lease, term of contract	72 months
Area of operating lease \| ft²	16,401
Total leased square feet of office space \| ft²	77,818
Initial monthly lease payments per month	$ 300,000
Percentage increase to existing per month rent payment	20.00%
Base rent payable	$ 19,400,000
Lease commencement date	Jan. 01, 2021

Equity Incentive Plans - Additional Information (Detail) - USD ($)	1 Months Ended		12 Months Ended
	Oct. 31, 2017	Aug. 31, 2017	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 31, 2021
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance			9,526,000	7,291,000	10,032,000
Stock options vesting period			48 months
Stock options vesting description			Options granted to employees on or after December 5, 2013 generally vest in installments of (i) 25% at the one-year anniversary and (ii) in either 36 equal monthly or 12 equal quarterly installments beginning in the thirteenth month after the initial vesting commencement date (as defined) subject to the employee’s continuous service with the Company.
Aggregate number of stock options granted to purchase common stock			2,884,000	4,400,000	2,100,000
Weighted - Average Grant Date Fair Value Per Option, Granted			$ 27.52	$ 17.31	$ 13.13
Aggregate intrinsic value of options exercised in period			$ 38,000,000.0	$ 15,700,000	$ 5,000,000.0
Fair market value of options vested in period			33,600,000	$ 12,900,000	5,100,000
Unrecognized compensation expense			$ 100,100,000
Estimated weighted-average period to recognize			2 years 10 months 2 days
Common stock, issued			76,130,000	63,938,000
Common stock issued at average per share price			$ 7.80
Share-based compensation expense			$ 45,376	$ 21,144	$ 7,733
Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options vesting period			4 years
Unrecognized compensation expense			$ 16,200,000
Estimated weighted-average period to recognize			3 years 4 months 24 days
Frist Anniversary [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options vesting percentage			25.00%
Second Anniversary [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options vesting percentage			25.00%
Third Anniversary [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options vesting percentage			25.00%
Fourth Anniversary [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options vesting percentage			25.00%
Equity Incentive Plan 2010 [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock shares reserved for issuance		6,188,466
Percentage of voting shares		10.00%
Options expire from issuance date		10 years
Equity Incentive Plan 2010 [Member] \| Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of estimated fair value per share of common stock on the date of grant		100.00%
Equity Incentive Plan 2010 [Member] \| Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage price of each share of the fair value on date of grant		110.00%
2017 Stock Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock shares reserved for issuance	1,359,587
Percentage of voting shares	4.00%
Number of shares available for future grant	299,568
Shares issuable increased during the period	4,219,409
Common Stock Issuance Description			equal to the lowest of 4,219,409 shares of common stock, 4.0% of the number of shares of common stock outstanding on the first day of the fiscal year and an amount determined by the board of directors.
Common stock reserved for future issuance			8,613,049	6,319,000	4,033,000
2017 Stock Incentive Plan [Member] \| Subsequent Event [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares available for future grant						3,045,189
2020 Inducement Stock Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance			750,000
2020 Inducement Stock Incentive Plan [Member] \| Subsequent Event [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance						1,050,000
2017 Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock shares reserved for issuance	468,823
Percentage of voting shares			1.00%
2017 Employee Stock Purchase Plan [Member] \| Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance			937,646
Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage price of each share of the fair value on date of grant			85.00%
Number of shares available for future grant			913,226
Percentage of earnings withheld to purchase shares of common stock			15.00%
Common stock, issued			58,938
Common stock issued at average per share price			$ 26.71
Cash received from issuance of purchase rights			$ 1,600,000
Share-based compensation expense			$ 800,000

Equity Incentive Plans - Summary of Share-based Compensation Expense (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 45,376	$ 21,144	$ 7,733
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	21,381	10,683	3,559
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 23,995	$ 10,461	$ 4,174

Equity Incentive Plans - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Shares Outstanding, Beginning balance	10,855,000
Shares, Granted	2,884,000	4,400,000	2,100,000
Shares, Exercised	(1,208,000)
Shares, Forfeited	(795,000)
Shares Outstanding, Ending balance	11,736,000	10,855,000
Shares, Options exercisable	6,062,000
Shares, Expected to vest	5,674,000
Weighted -Average Exercise Price Per Share Outstanding, Beginning balance	$ 13.52
Weighted -Average Exercise Price Per Share, Granted	37.93
Weighted -Average Exercise Price Per Share, Exercised	7.80
Weighted -Average Exercise Price Per Share, Forfeited	28.03
Weighted -Average Exercise Price Per Share, Outstanding, Ending balance	19.13	$ 13.52
Weighted -Average Exercise Price Per Share, Options exercisable	10.96
Weighted -Average Exercise Price Per Share, Expected to vest	27.85
Weighted - Average Grant Date Fair Value Per Option, Granted	$ 27.52	$ 17.31	$ 13.13
Weighted - Average Contractual Life, Options exercisable	6 years 2 months 19 days
Weighted - Average Contractual Life, Expected to vest	8 years 7 months 13 days
Aggregate Intrinsic Value, Options exercisable	$ 280,290
Aggregate Intrinsic Value, Expected to vest	$ 166,527

Equity Incentive Plans - Summary of Assumption Used to Estimate the Grant Date Fair Value (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Risk-free interest rate, minimum	0.32%	1.42%	2.68%
Risk-free interest rate, maximum	1.76%	2.56%	2.99%
Dividend yield	0.00%	0.00%	0.00%
Volatility, minimum	84.40%	102.60%	87.00%
Volatility, maximum	87.80%	111.20%	110.90%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Expected terms (years)	5 years 3 months 21 days	5 years 3 months 21 days	5 years 11 months 8 days
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Expected terms (years)	6 years 29 days	6 years 29 days	6 years 3 months

Equity Incentive Plans - Schedule of Unvested Restricted Stock Units Activity (Detail) - Restricted Stock Units [Member]	12 Months Ended
	Dec. 31, 2020 $ / shares shares
Number of Stock Units
Granted \| shares	546,000
Forfeited \| shares	(44,000)
Unvested Ending Balance \| shares	502,000
Weighted Average Grant Date Fair Value Per Share
Granted \| $ / shares	$ 38.97
Forfeited \| $ / shares	40.43
Unvested Balance at December 31, 2020 \| $ / shares	$ 38.84

Net Loss per Common Share - Summary of Basic and Diluted Net Loss per Common Share (Detail) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands

3 Months Ended

12 Months Ended

Dec. 31, 2020

Sep. 30, 2020

Jun. 30, 2020

Mar. 31, 2020

Dec. 31, 2019

Sep. 30, 2019

Jun. 30, 2019

Mar. 31, 2019

Dec. 31, 2020

Dec. 31, 2019

Dec. 31, 2018

Numerator:

Net loss

$ 78,265

$ (135,700)

$ (118,617)

$ (168,822)

$ (113,218)

$ (69,825)

$ (71,090)

$ (50,574)

$ (344,874)

$ (304,707)

$ (127,502)

Denominator:

75,163

62,229

54,396

Net loss per common share, basic and diluted

$ (1.77)

^[1]

$ (1.10)

^[1]

$ (1.12)

^[1]

$ (0.87)

^[1]

$ (4.59)

$ (4.90)

$ (2.34)

[1]

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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Quarterly Financial Information Disclosure [Abstract]

Net Loss per Common Share - Summary of Shares Outstanding that were Excluded from Calculation of Diluted Net Loss Per Share (Detail) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount	25,415	10,854	7,512
Convertible Notes [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount	13,177
Common Stock Under Option [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount	11,736	10,854	7,498
Restricted Stock Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount	502
Common Stock Warrants [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount			14

Related Party Transaction - Additional Information (Detail) - Revon [Member] - License Agreement and Services Agreement [Member] - USD ($)		12 Months Ended
	May 01, 2018	Dec. 31, 2020	Dec. 31, 2019
Related Party Transaction [Line Items]
Annual license fee	$ 200,000
Exclusive license option, thereafter	400,000
Development service costs	$ 300,000
Related party transaction, amount paid		$ 0	$ 700,000

Selected Quarterly Financial Data (Unaudited) - Summary of Interim Financial Information on Quarterly Basis (Detail) - USD ($)
$ / shares in Units, $ in Thousands

3 Months Ended

12 Months Ended

Dec. 31, 2020

Sep. 30, 2020

Jun. 30, 2020

Mar. 31, 2020

Dec. 31, 2019

Sep. 30, 2019

Jun. 30, 2019

Mar. 31, 2019

Dec. 31, 2020

Dec. 31, 2019

Dec. 31, 2018

Operating income/ (loss)

$ 130,120

$ (129,552)

$ (115,508)

$ (98,786)

$ (105,940)

$ (69,948)

$ (63,476)

$ (48,651)

$ (213,726)

$ (288,015)

$ (127,925)

Net loss

$ 78,265

$ (135,700)

$ (118,617)

$ (168,822)

$ (113,218)

$ (69,825)

$ (71,090)

$ (50,574)

$ (344,874)

$ (304,707)

$ (127,502)

Net income/(loss) per common share, basic

^[1]

$ 1.03

$ (1.79)

$ (1.57)

$ (2.29)

Net income/(loss) per common share, diluted

^[1]

$ 0.93

$ (1.79)

$ (1.57)

$ (2.29)

Net Loss per common share, basic and diluted

$ (1.77)

^[1]

$ (1.10)

^[1]

$ (1.12)

^[1]

$ (0.87)

^[1]

$ (4.59)

$ (4.90)

$ (2.34)

[1]

The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.