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Document and Entity Information - shares	6 Months Ended
Document and Entity Information - shares	Jun. 30, 2018	Jul. 20, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q2
Trading Symbol	APLS
Entity Registrant Name	Apellis Pharmaceuticals, Inc.
Entity Central Index Key	0001492422
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		56,178,124

Condensed Consolidated Balance Sheets - USD ($)	Jun. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 253,839,540	$ 175,643,529
Refundable research and development credit	1,996,996	1,297,361
Prepaid assets	10,857,287	5,059,593
Other current assets	7,695	14,823
Total current assets	266,701,518	182,015,306
Other assets	116,150	116,150
Total assets	266,817,668	182,131,456
Current liabilities:
Accounts payable	5,133,017	3,663,253
Accrued expenses	5,121,178	2,890,705
Total current liabilities	10,254,195	6,553,958
Long-term liabilities:
Term loan facility	20,143,217	19,806,944
Promissory note - related party	6,618,150	6,583,402
Common stock warrant liability	250,000	244,292
Total liabilities	37,265,562	33,188,596
Stockholders' equity:
Preferred stock, $0.0001 par value; 10,000,000 shares authorized, and zero shares issued and outstanding at December 31, 2017 and June 30, 2018	0	0
Common stock, $0.0001 par value; 200,000,000 shares authorized at December 31, 2017 and June 30, 2018 and 50,334,152 shares issued and outstanding at December 31, 2017 and 56,154,438 shares issued and outstanding at June 30, 2018	5,615	5,033
Additional paid in capital	433,880,750	298,201,480
Accumulated deficit	(204,334,259)	(149,263,653)
Total stockholders' equity	229,552,106	148,942,860
Total liabilities and stockholders' equity	$ 266,817,668	$ 182,131,456

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Jun. 30, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, issued	0	0
Preferred stock, outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, issued	56,154,438	50,334,152
Common stock, outstanding	56,154,438	50,334,152

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Operating expenses:
Research and development	$ 27,537,619	$ 10,422,059	$ 44,940,509	$ 17,653,143
General and administrative	5,947,823	1,710,344	9,983,079	3,531,753
Operating loss	(33,485,442)	(12,132,403)	(54,923,588)	(21,184,896)
Interest income (expense), net	184,009	5,854	(82,677)	15,966
Other income (expense), net	(32,867)	397	(64,341)	(7,995)
Net loss and comprehensive loss	$ (33,334,300)	$ (12,126,152)	$ (55,070,606)	$ (21,176,925)
Net loss per common share, basic and diluted	$ (0.61)	$ (1.44)	$ (1.05)	$ (2.51)
Weighted-average number of common shares used in net loss per common share, basic and diluted	54,691,833	8,428,366	52,534,806	8,428,366

Condensed Consolidated Statements of Changes in Stockholder's Equity - 6 months ended Jun. 30, 2018 - USD ($)	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2017	$ 148,942,860	$ 5,033	$ 298,201,480	$ (149,263,653)
Beginning balance, Shares at Dec. 31, 2017		50,334,152
Issuance of common stock in follow-on offering, net of offering costs	131,325,027	$ 550	131,324,477
Issuance of common stock in follow-on offering, net of offering costs, Shares		5,500,000
Issuance of common stock upon exercise of stock options	501,110	$ 32	501,078
Issuance of common stock upon exercise of stock options, Shares		320,286
Share-based compensation expense	3,853,715		3,853,715
Net loss	(55,070,606)			(55,070,606)
Ending balance at Jun. 30, 2018	$ 229,552,106	$ 5,615	$ 433,880,750	$ (204,334,259)
Ending balance, Shares at Jun. 30, 2018		56,154,438

Condensed Consolidated Statements of Cash Flows - USD ($)	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Operating Activities
Net loss	$ (55,070,606)	$ (21,176,925)
Adjustments to reconcile net loss to net cash used in operating activities:
Share-based compensation expense	3,853,715	708,833
Loss from remeasurement of fair value of warrants	5,708
Accretion of discounts	34,748
Amortization of term loan facility	336,273
Changes in operating assets and liabilities:
Refundable research and development credit	(699,635)	(416,412)
Prepaid assets	(5,797,694)	(1,457,696)
Other current assets	7,128
Other assets		(61,632)
Accounts payable	1,469,764	57,186
Accrued expenses	2,230,473	2,422,245
Net cash used in operating activities	(53,630,126)	(19,924,401)
Financing Activities
Proceeds from issuance of common stock, net of issuance costs	131,325,027
Proceeds from exercise of stock options	501,110
Net cash provided by financing activities	131,826,137
Net decrease in cash and cash equivalents	78,196,011	(19,924,401)
Cash and cash equivalents at beginning of period	175,643,529	24,863,488
Cash and cash equivalents at end of period	$ 253,839,540	$ 4,939,087

Nature of Organization and Operations	6 Months Ended
Nature of Organization and Operations	Jun. 30, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of Organization and Operations	1. Nature of Organization and Operations Apellis Pharmaceuticals, Inc. (the “Company”) is a clinical-stage biopharmaceutical company focused on the development of novel therapeutic compounds to treat disease through the inhibition of the complement system, which is an integral component of the immune system, at the level of C3, the central protein in the complement cascade. The Company was incorporated in September 2009 under the laws of the State of Delaware and has its principal office in Crestwood, Kentucky. The Company’s operations since inception have been limited to organizing and staffing the Company, acquiring rights to product candidates, business planning, raising capital and developing its product candidates. The Company is subject to risks common in the biotechnology industry including, but not limited to, raising additional capital, development by its competitors of new technological innovations, its ability to successfully complete preclinical and clinical development of product candidates and receive timely regulatory approval of products, market acceptance of the Company’s products, protection of proprietary technology, healthcare cost containment initiatives, and compliance with governmental regulations, including those of the U.S. Food and Drug Administration (“FDA”). Liquidity and Financial Condition The accompanying unaudited condensed consolidated financial statements have been prepared on the basis of the realization of assets and the satisfaction of liabilities and commitments in the normal course of business. As of July 31, 2018, the date of issuance of these unaudited condensed consolidated financial statements, the Company believes that its cash and cash equivalents as of June 30, 2018 of $253.8 million will be sufficient to fund its operations for at least twelve months into the third quarter of 2019. The future viability beyond that point is dependent on its ability to raise additional capital to finance its operations. The Company is subject to risks common to other life science companies in the development stage including, but not limited to, uncertainty of product development and commercialization, lack of marketing and sales history, development by its competitors of new technological innovations, dependence on key personnel, market acceptance of products, product liability, protection of proprietary technology, ability to raise additional financing, and compliance with FDA and other government regulations. If the Company does not successfully commercialize any of its product candidates, it will be unable to generate recurring product revenue or achieve profitability. Management’s plans in order to meet its short-term and longer term operating cash flow requirements include obtaining additional funding. There are uncertainties associated with the Company’s ability to (1) obtain additional debt or equity financing (2) enter into collaborative agreements with strategic partners, and (3) succeed in its future operations. If the Company is not able to obtain the required capital to fund its operations from any of these, or is not able to obtain such funding on terms that are favorable to the Company, it could be forced to delay, reduce or eliminate its research and development programs or future commercialization efforts and its business could be materially harmed.

Basis of Presentation	6 Months Ended
Basis of Presentation	Jun. 30, 2018
Accounting Policies [Abstract]
Basis of Presentation	2. Basis of Presentation The accompanying unaudited condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiary, Apellis Australia Pty Ltd. All intercompany balances and transactions have been eliminated in consolidation. The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“U.S. GAAP”) and following the requirements of the Securities and Exchange Commission (the “SEC”), for interim reporting. As permitted under those rules, certain footnotes or other financial information that are normally required by U.S. GAAP have been condensed or omitted and, accordingly, the balance sheet as of December 31, 2017 has been derived from audited consolidated financial statements at that date but does not include all of the information required by U.S. GAAP for complete financial statements. These financial statements have been prepared on the same basis as the Company’s annual financial statements and, in the opinion of management, reflect all adjustments (consisting only of normal recurring adjustments) that are necessary for a fair presentation of the Company’s financial information. The results of operations for the six months ended June 30, 2018 are not necessarily indicative of the results to be expected for the year ending December 31, 2018 or for any other interim period or for any other future year. The accompanying unaudited condensed consolidated financial statements and related financial information should be read in conjunction with the audited consolidated financial statements and the related notes thereto for the year ended December 31, 2017 included in the Company’s Annual Report on Form 10-K filed with the Securities and Exchange Commission on March 19, 2018. Reverse Stock Split On October 27, 2017, the Company effectuated a l-for-2.133 reverse stock split of its outstanding common stock, which was approved by the Company’s board of directors on October 27, 2017. The reverse stock split resulted in an adjustment to the conversion prices of its convertible preferred stock to reflect a proportional decrease in the number of shares of common stock to be issued upon conversion. The accompanying unaudited condensed consolidated financial statements and notes to the financial statements give retroactive effect to the reverse stock split for all periods presented. Follow-on Offering On April 23, 2018, the Company issued and sold 5,500,000 shares of its common stock at a price per share of $25.50 in a follow-on public offering. The Company received net proceeds of $131.3 million after deducting underwriting discounts and commissions of $8.4 million and offering costs of $0.5 million. Initial Public Offering On November 13, 2017, the Company issued and sold 10,714,000 shares of its common stock at a price per share of $14.00 in its initial public offering (the “IPO”). The Company received net proceeds of $137.2 million after deducting underwriting discounts and commissions of $10.5 million and offering costs of $2.3 million. In addition, on December 13, 2017, the Company issued and sold an additional 981,107 shares of its common stock at the initial public offering price of $14.00 per share pursuant to the underwriters’ partial exercise of their option to purchase additional shares of common stock, which resulted in net proceeds of approximately $12.8 million, after underwriting discounts and commissions of $1.0 million. Automatic Conversion of Preferred Stock On November 13, 2017, upon the closing of the Company’s IPO, all outstanding shares of the Company’s convertible preferred stock converted into 30,070,034 shares of the Company’s common stock. Fair Value of Financial Instruments Management believes that the carrying amounts of the Company’s financial instruments, including accounts payable and accrued expenses, approximate the fair value due to the short-term nature of those instruments. The Company follows the fair value hierarchy within Accounting Standards Codification (“ASC”) 820, Fair Value Measurements, and classifies its financial instruments as Level I. Recent Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 2014-09, Revenue from Contracts with Customers (Topic 606). The standard applies one comprehensive revenue recognition model across all contracts, entities and sectors. The core principle of the new standard is that revenue should be recognized to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. As the Company does not currently have revenue, the adoption of the new standard had no impact on its consolidated financial statements. In February 2016, the FASB issued ASU 2016-02, Leases, which establishes a comprehensive new lease accounting model. The new standard: (a) clarifies the definition of a lease; (b) requires a dual approach to lease classification similar to current lease classifications; and, (c) causes lessees to recognize leases on the balance sheet as a lease liability with a corresponding right-of-use asset for leases with a lease-term of more than twelve months. The new standard is effective for fiscal years and interim periods beginning after December 15, 2018 and requires modified retrospective application. Early adoption is permitted. The Company is currently evaluating the impact that the adoption of ASU 2016-02 will have on its consolidated financial statements. In June 2018, the FASB issued ASU No. 2018-07, Compensation - Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting. These amendments expand the scope of Topic 718, Compensation - Stock Compensation (which currently only includes share-based payments to employees) to include share-based payments issued to nonemployees for goods or services. Consequently, the accounting for share-based payments to nonemployees and employees will be substantially aligned. The ASU supersedes Subtopic 505-50, Equity - Equity-Based Payments to Non-Employees. The guidance is effective for public companies for fiscal years, and interim fiscal periods within those fiscal years, beginning after December 15, 2018. Early adoption is permitted, but no earlier than a company’s adoption date of Topic 606, Revenue from Contracts with Customers. The Company is assessing ASU 2018-07 and does not expect it to have a material impact on its accounting and disclosures.

Convertible Preferred Stock	6 Months Ended
Convertible Preferred Stock	Jun. 30, 2018
Equity [Abstract]
Convertible Preferred Stock	3. Convertible Preferred Stock Series E Convertible Preferred Stock In August 2017, the Company issued and sold 7,792,035 shares of series E convertible preferred stock at $2.571 per share for aggregate proceeds of $20,033,322, less issuance costs of $285,475. Upon closing of the Company’s IPO on November 13, 2017, all shares of convertible preferred stock converted into shares of common stock.

Accrued Expenses

6 Months Ended

Jun. 30, 2018

Text Block [Abstract]

Accrued Expenses

4. Accrued Expenses

Accrued expenses are as follows:

	December 31,		June 30,
	2017		2018
Accrued research and development	$	2,463,808	$	3,478,400
Accrued payroll liabilities		251,491		1,461,919
Other		175,406		180,859
Total	$	2,890,705	$	5,121,178

Long-term Debt

6 Months Ended

Jun. 30, 2018

Debt Disclosure [Abstract]

Long-term Debt

5. Long term Debt

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 bear interest at a floating rate per annum equal to the WSJ prime rate plus 1.50%; provided, however, that in an event of default, as defined in the loan and security agreement, the interest rate applicable to borrowings under such agreement will be increased by 5.0%. Under the agreement, the Company is required to make monthly interest-only payments through November 1, 2019 and is required to make 24 equal monthly payments of principal, plus accrued interest, from November 1, 2019 through October 1, 2021, when all unpaid principal and interest becomes due and payable.

The Company may voluntarily prepay all, but not less than all, of the outstanding principal at any time prior to the maturity date, subject to a prepayment fee, which ranges from 0% to 2% of the outstanding principal. A final payment of $1.6 million is due upon the earlier to occur of the maturity of the loan, the acceleration or prepayment of all outstanding principal or the termination of the term loan facility.

Borrowings under the term loan facility are secured by a first priority lien on all of the Company’s assets, excluding intellectual property owned by the Company. The Company has agreed to a negative pledge on its intellectual property and to grant a security interest on its interest in its licenses from the University of Pennsylvania (“Penn”) if Penn consents to such security interest. The term loan facility contains customary events of default and affirmative and negative covenants, including restrictions on the Company’s ability to pay dividends and incur additional debt, but does not contain any financial covenants.

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 has a ten-year term and includes 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 may require the Company to repurchase the warrant for a total aggregate purchase price of $250,000.

Related Party 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”), an affiliate of one of the Company’s stockholders. The promissory note bears interest at a rate per annum of 8.0%, and is due and payable quarterly in arrears on the 19th day of each April, July, October and January beginning on January 19, 2018. The promissory note has a maturity date of October 19, 2022. The promissory note is contractually subordinated to the term loan facility with SVB.

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 $430,160 as a discount to the promissory note. This amount is being accreted as additional interest expense over the term of the promissory note.

The contractual maturities of our long term debt obligations due subsequent to June 30, 2018 are as follows:

2018	$	—
2019		1,666,667
2020		10,000,000
2021		8,333,333
2022		7,000,000
	$	27,000,000

Refundable Research and Development Credit and Income Taxes	6 Months Ended
	Jun. 30, 2018
Text Block [Abstract]
Refundable Research And Development Credit And Income Taxes	6. Refundable Research and Development Credit and Income Taxes The Company earns non-income related refundable Australian research and development credits that are settled and paid to the Company annually. The associated income from the credits are an offset to research and development expenses. The Company’s income tax provision is computed based on the federal statutory rate and the average state statutory rates, net of the related federal benefit. For the three months ended June 30, 2018 and 2017, there were no current or deferred income tax expenses or benefits due to the Company’s, net losses, research and development credits and increases in its deferred tax asset valuation allowance during those periods. The Company’s estimate of the realizability of the deferred tax asset is dependent on estimates of projected future levels of taxable income. In analyzing future taxable income levels, the Company considered all evidence currently available, both positive and negative. Based on this analysis, the Company has recorded a valuation allowance for all deferred tax assets as of June 30, 2018 and December 31, 2017. Tax Act Enactment During the fourth quarter of 2017, the Company accounted for the estimated impacts of the Tax Cuts and Jobs Act of 2017 (the “Tax Act”) including the re-measurement of the Company’s deferred tax assets and liabilities, which resulted in a total impact of $16.0 million to the gross deferred tax balance. As there is a valuation allowance recorded for all deferred tax assets as of, December 31, 2017 there was no impact to the net deferred tax assets balance. The Company was not subject to the one-time mandatory transition tax on cumulative foreign subsidiary earnings as the Company has a net cumulative deficit in earnings and profits from foreign subsidiaries. As of June 30, 2018, the estimated impacts of the Tax Act recorded remain provisional in nature, and the Company will continue to assess the impact that various provisions of the Tax Act will have on its business. While the amounts are provisional, given the full valuation allowance as of June 30, 2018, the Company does not anticipate an overall impact to the net deferred tax asset balance.

Commitments and Contingencies	6 Months Ended
Commitments and Contingencies	Jun. 30, 2018
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	7. Commitments and Contingencies The Company contracts to conduct research and development activities with remaining contract costs of approximately $1,162,000 at June 30, 2018, that will be incurred in future periods. 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.

Net Loss per Share

6 Months Ended

Jun. 30, 2018

Earnings Per Share [Abstract]

Net Loss per Share

8. Net Loss per Share

Since the Company was in a loss position for all periods presented, basic net loss per common share is the same as diluted net loss per common share for all periods presented as the inclusion of all potential common shares outstanding would have been anti-dilutive. 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:

	For the Three Months Ended June 30,		For the Six Months Ended June 30,
	2017	2018	2017	2018
Convertible preferred stock	26,416,896	—	26,416,896	—
Common stock options	4,675,118	7,293,336	4,675,118	7,293,336
Common stock warrants	—	14,064	—	14,064
Total	31,092,014	7,307,400	31,092,014	7,307,400

Related Party Transaction	6 Months Ended
Related Party Transaction	Jun. 30, 2018
Related Party Transactions [Abstract]
Related Party Transaction	9. 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 $175,000 and an option to obtain a perpetual, exclusive license thereafter for $350,000. Under the services agreement, Revon will provide development services with respect to the Revon software to the Company for $250,000 during the first year. Each of Cedric Francois, Pascal Deschatelets and Alec Machiels is 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.

Summary of Significant Accounting Policies (Policies)	6 Months Ended
Summary of Significant Accounting Policies (Policies)	Jun. 30, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiary, Apellis Australia Pty Ltd. All intercompany balances and transactions have been eliminated in consolidation. The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“U.S. GAAP”) and following the requirements of the Securities and Exchange Commission (the “SEC”), for interim reporting. As permitted under those rules, certain footnotes or other financial information that are normally required by U.S. GAAP have been condensed or omitted and, accordingly, the balance sheet as of December 31, 2017 has been derived from audited consolidated financial statements at that date but does not include all of the information required by U.S. GAAP for complete financial statements. These financial statements have been prepared on the same basis as the Company’s annual financial statements and, in the opinion of management, reflect all adjustments (consisting only of normal recurring adjustments) that are necessary for a fair presentation of the Company’s financial information. The results of operations for the six months ended June 30, 2018 are not necessarily indicative of the results to be expected for the year ending December 31, 2018 or for any other interim period or for any other future year. The accompanying unaudited condensed consolidated financial statements and related financial information should be read in conjunction with the audited consolidated financial statements and the related notes thereto for the year ended December 31, 2017 included in the Company’s Annual Report on Form 10-K filed with the Securities and Exchange Commission on March 19, 2018.
Reverse Stock Split	Reverse Stock Split On October 27, 2017, the Company effectuated a l-for-2.133 reverse stock split of its outstanding common stock, which was approved by the Company’s board of directors on October 27, 2017. The reverse stock split resulted in an adjustment to the conversion prices of its convertible preferred stock to reflect a proportional decrease in the number of shares of common stock to be issued upon conversion. The accompanying unaudited condensed consolidated financial statements and notes to the financial statements give retroactive effect to the reverse stock split for all periods presented.
Follow-on Offering	Follow-on Offering On April 23, 2018, the Company issued and sold 5,500,000 shares of its common stock at a price per share of $25.50 in a follow-on public offering. The Company received net proceeds of $131.3 million after deducting underwriting discounts and commissions of $8.4 million and offering costs of $0.5 million.
Initial Public Offering	Initial Public Offering On November 13, 2017, the Company issued and sold 10,714,000 shares of its common stock at a price per share of $14.00 in its initial public offering (the “IPO”). The Company received net proceeds of $137.2 million after deducting underwriting discounts and commissions of $10.5 million and offering costs of $2.3 million. In addition, on December 13, 2017, the Company issued and sold an additional 981,107 shares of its common stock at the initial public offering price of $14.00 per share pursuant to the underwriters’ partial exercise of their option to purchase additional shares of common stock, which resulted in net proceeds of approximately $12.8 million, after underwriting discounts and commissions of $1.0 million.
Automatic Conversion of Preferred Stock	Automatic Conversion of Preferred Stock On November 13, 2017, upon the closing of the Company’s IPO, all outstanding shares of the Company’s convertible preferred stock converted into 30,070,034 shares of the Company’s common stock.
Fair Value of Financial Instruments	Fair Value of Financial Instruments Management believes that the carrying amounts of the Company’s financial instruments, including accounts payable and accrued expenses, approximate the fair value due to the short-term nature of those instruments. The Company follows the fair value hierarchy within Accounting Standards Codification (“ASC”) 820, Fair Value Measurements, and classifies its financial instruments as Level I.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 2014-09, Revenue from Contracts with Customers (Topic 606). The standard applies one comprehensive revenue recognition model across all contracts, entities and sectors. The core principle of the new standard is that revenue should be recognized to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. As the Company does not currently have revenue, the adoption of the new standard had no impact on its consolidated financial statements. In February 2016, the FASB issued ASU 2016-02, Leases, which establishes a comprehensive new lease accounting model. The new standard: (a) clarifies the definition of a lease; (b) requires a dual approach to lease classification similar to current lease classifications; and, (c) causes lessees to recognize leases on the balance sheet as a lease liability with a corresponding right-of-use asset for leases with a lease-term of more than twelve months. The new standard is effective for fiscal years and interim periods beginning after December 15, 2018 and requires modified retrospective application. Early adoption is permitted. The Company is currently evaluating the impact that the adoption of ASU 2016-02 will have on its consolidated financial statements. In June 2018, the FASB issued ASU No. 2018-07, Compensation - Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting. These amendments expand the scope of Topic 718, Compensation - Stock Compensation (which currently only includes share-based payments to employees) to include share-based payments issued to nonemployees for goods or services. Consequently, the accounting for share-based payments to nonemployees and employees will be substantially aligned. The ASU supersedes Subtopic 505-50, Equity - Equity-Based Payments to Non-Employees. The guidance is effective for public companies for fiscal years, and interim fiscal periods within those fiscal years, beginning after December 15, 2018. Early adoption is permitted, but no earlier than a company’s adoption date of Topic 606, Revenue from Contracts with Customers. The Company is assessing ASU 2018-07 and does not expect it to have a material impact on its accounting and disclosures.

Accrued Expenses (Tables)

6 Months Ended

Jun. 30, 2018

Text Block [Abstract]

Schedule of Accrued Expenses

Accrued expenses are as follows:

	December 31,		June 30,
	2017		2018
Accrued research and development	$	2,463,808	$	3,478,400
Accrued payroll liabilities		251,491		1,461,919
Other		175,406		180,859
Total	$	2,890,705	$	5,121,178

Long-term Debt (Tables)

6 Months Ended

Jun. 30, 2018

Debt Disclosure [Abstract]

Summary of Contractual Maturities of Long-term Debt Obligations

The contractual maturities of our long term debt obligations due subsequent to June 30, 2018 are as follows:

2018	$	—
2019		1,666,667
2020		10,000,000
2021		8,333,333
2022		7,000,000
	$	27,000,000

Net Loss per Share (Tables)

6 Months Ended

Jun. 30, 2018

Earnings Per Share [Abstract]

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:

	For the Three Months Ended June 30,		For the Six Months Ended June 30,
	2017	2018	2017	2018
Convertible preferred stock	26,416,896	—	26,416,896	—
Common stock options	4,675,118	7,293,336	4,675,118	7,293,336
Common stock warrants	—	14,064	—	14,064
Total	31,092,014	7,307,400	31,092,014	7,307,400

Nature of Organization and Operations - Additional Information (Detail) - USD ($)	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Cash and cash equivalents	$ 253,839,540	$ 175,643,529	$ 4,939,087	$ 24,863,488

Basis of Presentation - Additional Information (Detail)					6 Months Ended
Basis of Presentation - Additional Information (Detail)	Apr. 23, 2018 USD ($) $ / shares shares	Dec. 13, 2017 USD ($) $ / shares shares	Nov. 13, 2017 USD ($) $ / shares shares	Oct. 27, 2017	Jun. 30, 2018 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Description of common stock reverse stock split					The Company effectuated a l-for-2.133 reverse stock split of its outstanding common stock, which was approved by the Company’s board of directors on October 27, 2017.
Common stock reverse stock split, conversion ratio				0.468823254
Reverse stock split, effective date				Oct. 27, 2017
Proceeds from follow-on public offering after deducting underwriting discounts and commissions					$ 131,325,027
Conversion of preferred stock to common stock \| shares			30,070,034
Follow-on Offering [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of additional shares issued and sold \| shares	5,500,000
Price of common stock \| $ / shares	$ 25.50
Proceeds from follow-on public offering after deducting underwriting discounts and commissions	$ 131,300,000
Underwriting discounts and commissions	8,400,000
Offering cost	$ 500,000
IPO [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of additional shares issued and sold \| shares		981,107	10,714,000
Price of common stock \| $ / shares		$ 14.00	$ 14.00
Underwriting discounts and commissions		$ 1,000,000	$ 10,500,000
Offering cost			2,300,000
Proceeds from initial offering after deducting underwriting discounts and commissions		$ 12,800,000	$ 137,200,000

Convertible Preferred Stock - Additional Information (Detail) - USD ($)	1 Months Ended
	Aug. 31, 2017	Jun. 30, 2018	Dec. 31, 2017
Class Of Stock [Line Items]
Convertible preferred stock issued and sold		0	0
Preferred stock, par value		$ 0.0001	$ 0.0001
Series E Convertible Preferred Stock [Member]
Class Of Stock [Line Items]
Convertible preferred stock issued and sold	7,792,035
Preferred stock, par value	$ 2.571
Gross proceeds from issuance and sale of convertible preferred stock	$ 20,033,322
Stock issuance costs	$ 285,475

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($)	Jun. 30, 2018	Dec. 31, 2017
Accrued Liabilities Current [Abstract]
Accrued research and development	$ 3,478,400	$ 2,463,808
Accrued payroll liabilities	1,461,919	251,491
Other	180,859	175,406
Total	$ 5,121,178	$ 2,890,705

Long-term Debt - Additional Information (Detail)			6 Months Ended
Long-term Debt - Additional Information (Detail)	Oct. 20, 2017 USD ($) Installment $ / shares shares	Oct. 19, 2017 USD ($) $ / shares shares	Jun. 30, 2018
Unsecured Promissory Note \| Golda Darty Partners S A [Member]
Debt Instrument [Line Items]
Debt instrument, due and payment description			The promissory note bears interest at a rate per annum of 8.0%, and is due and payable quarterly in arrears on the 19th day of each April, July, October and January beginning on January 19, 2018.
Warrant to purchase common stock \| shares		93,764
Exercise price per share \| $ / shares		$ 5.484
Principal amount		$ 7,000,000
Interest rate		8.00%
Maturity date		Oct. 19, 2022
Fair value of warrant		$ 430,160
Term Loan Facility [Member]
Debt Instrument [Line Items]
Final payment due amount	$ 1,600,000
Term Loan Facility [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Prepayment fee, percentage	0.00%
Term Loan Facility [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Prepayment fee, percentage	2.00%
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	$ 250,000
Loan and Security Agreement [Member] \| Silicon Valley Bank [Member]
Debt Instrument [Line Items]
Term loan facility	$ 20,000,000
Increase in interest rate	5.00%
Debt instrument, due and payment description			The Company is required to make monthly interest-only payments through November 1, 2019 and is required to make 24 equal monthly payments of principal, plus accrued interest, from November 1, 2019 through October 1, 2021, when all unpaid principal and interest becomes due and payable
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%

Long-term Debt - Summary of Contractual Maturities of Long-term Debt Obligations (Detail)	Jun. 30, 2018 USD ($)
Debt Disclosure [Abstract]
2019	$ 1,666,667
2020	10,000,000
2021	8,333,333
2022	7,000,000
Long-term Debt	$ 27,000,000

Refundable Research and Development Credit and Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Income Tax Expense Benefit Continuing Operations [Abstract]
Current income tax expense or benefits	$ 0		$ 0
Deferred income tax expense or benefits	$ 0		$ 0
Gross deferred tax balance		$ 16,000,000

Commitments and Contingencies - Additional Information (Detail)	Jun. 30, 2018 USD ($)
Commitments And Contingencies Disclosure [Abstract]
Remaining contract costs	$ 1,162,000

Net Loss per Share - Summary of Shares Outstanding that were Excluded from Calculation of Diluted Net Loss Per Share (Detail) - shares	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount	7,307,400	31,092,014	7,307,400	31,092,014
Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount		26,416,896		26,416,896
Common Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount	7,293,336	4,675,118	7,293,336	4,675,118
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	14,064		14,064

Related Party Transaction - Additional Information (Detail) - Revon [Member] - License Agreement and Services Agreement [Member]	May 01, 2018 USD ($)
Related Party Transaction [Line Items]
Annual license fee	$ 175,000
Exclusive license option, thereafter	350,000
Development service costs	$ 250,000