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Document and Entity Information - shares	9 Months Ended
Document and Entity Information - shares	Sep. 30, 2017	Dec. 20, 2017
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2017
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	Q3
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		50,334,152

Condensed Consolidated Balance Sheets - USD ($)	Sep. 30, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 13,135,848	$ 24,863,488
Refundable research and development credit	2,243,565	1,347,804
Other current assets	3,653,520	1,157,438
Total current assets	19,032,933	27,368,730
Other assets	108,350	64,528
Total assets	19,141,283	27,433,258
Current liabilities:
Accounts payable	4,001,059	2,547,212
Accrued expenses	2,936,885	1,091,726
Total current liabilities	6,937,944	3,638,938
Stockholders' equity:
Common stock, $0.0001 par value; 87,000,000 shares authorized at December 31, 2016 and September 30, 2017; 8,428,366 shares issued and outstanding at December 31, 2016 and 8,472,903 shares issued and outstanding at September 30, 2017	847	843
Additional paid in capital	31,407,476	29,996,110
Accumulated deficit	(131,007,757)	(98,257,559)
Stockholders' equity	12,203,339	23,794,320
Total liabilities and stockholders' equity	19,141,283	27,433,258
Series A Convertible Preferred Stock [Member]
Stockholders' equity:
Preferred stock value	2,654,405	2,654,405
Stockholders' equity	2,654,405	2,654,405
Series B Convertible Preferred Stock [Member]
Stockholders' equity:
Preferred stock value	6,944,148	6,944,148
Stockholders' equity	6,944,148	6,944,148
Series C Convertible Preferred Stock [Member]
Stockholders' equity:
Preferred stock value	35,542,707	35,542,707
Stockholders' equity	35,542,707	35,542,707
Series D Convertible Preferred Stock [Member]
Stockholders' equity:
Preferred stock value	46,913,666	46,913,666
Stockholders' equity	46,913,666	$ 46,913,666
Series E Convertible Preferred Stock [Member]
Stockholders' equity:
Preferred stock value	19,747,847
Stockholders' equity	$ 19,747,847

Condensed Consolidated Balance Sheets (Parenthetical) - USD ($)	Sep. 30, 2017	Dec. 31, 2016
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	87,000,000	87,000,000
Common stock, issued	8,472,903	8,428,366
Common stock, outstanding	8,472,903	8,428,366
Series A Convertible Preferred Stock [Member]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	2,670,000	2,670,000
Preferred stock, issued	2,670,000	2,670,000
Preferred stock, outstanding	2,670,000	2,670,000
Preferred stock, liquidation value	$ 2,670,000
Series B Convertible Preferred Stock [Member]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	6,362,658	6,362,658
Preferred stock, issued	6,362,658	6,362,658
Preferred stock, outstanding	6,362,658	6,362,658
Preferred stock, liquidation value	$ 6,998,924
Series C Convertible Preferred Stock [Member]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	26,215,411	26,215,411
Preferred stock, issued	26,215,411	26,215,411
Preferred stock, outstanding	26,215,411	26,215,411
Preferred stock, liquidation value	$ 32,769,264
Series D Convertible Preferred Stock [Member]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	21,099,351	21,099,351
Preferred stock, issued	21,099,351	21,099,351
Preferred stock, outstanding	21,099,351	21,099,351
Preferred stock, liquidation value	$ 47,135,950
Series E Convertible Preferred Stock [Member]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	23,376,108
Preferred stock, issued	7,792,035
Preferred stock, outstanding	7,792,035
Preferred stock, liquidation value	$ 20,033,322

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Operating expenses:
Research and development	$ 9,517,882	$ 6,234,058	$ 27,171,025	$ 17,387,027
General and administrative	2,071,437	1,000,146	5,603,190	3,216,467
Operating loss	(11,589,319)	(7,234,204)	(32,774,215)	(20,603,494)
Other income	16,046	23,613	24,017	98,959
Net loss and comprehensive loss	$ (11,573,273)	$ (7,210,591)	$ (32,750,198)	$ (20,504,535)
Net loss per common share, basic and diluted	$ (1.37)	$ (0.86)	$ (3.88)	$ (2.43)
Weighted-average number of common shares used in net loss per common share, basic and diluted	8,442,072	8,428,366	8,432,983	8,428,366

Condensed Consolidated Statements of Changes in Stockholder's Equity - 9 months ended Sep. 30, 2017 - USD ($)	Total	Series A Convertible Preferred Stock [Member]	Series B Convertible Preferred Stock [Member]	Series C Convertible Preferred Stock [Member]	Series D Convertible Preferred Stock [Member]	Series E Convertible Preferred Stock [Member]	Common Stock [Member]	Paid-in Capital [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2016	$ 23,794,320	$ 2,654,405	$ 6,944,148	$ 35,542,707	$ 46,913,666		$ 843	$ 29,996,110	$ (98,257,559)
Beginning balance, Shares at Dec. 31, 2016		2,670,000	6,362,658	26,215,411	21,099,351		8,428,366
Issuance of Series E preferred stock, net of issuance costs	19,747,847					$ 19,747,847
Issuance of Series E preferred stock, net of issuance costs, shares						7,792,035
Exercise of stock options	$ 157,000						$ 4	156,996
Exercise of stock options, shares	44,537						44,537
Share-based compensation expense	$ 1,254,370							1,254,370
Net loss	(32,750,198)								(32,750,198)
Ending balance at Sep. 30, 2017	$ 12,203,339	$ 2,654,405	$ 6,944,148	$ 35,542,707	$ 46,913,666	$ 19,747,847	$ 847	$ 31,407,476	$ (131,007,757)
Ending balance, Shares at Sep. 30, 2017		2,670,000	6,362,658	26,215,411	21,099,351	7,792,035	8,472,903

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($)	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Operating Activities
Net loss	$ (32,750,198)	$ (20,504,535)
Adjustments to reconcile net loss to net cash used in operating activities:
Share-based compensation expense	1,254,370	762,429
Changes in operating assets and liabilities:
Refundable research and development credit	(895,761)	(953,751)
Other current assets	(1,925,864)	(277,885)
Other assets	(43,822)	3,407
Accounts payable	1,453,847	319,358
Accrued expenses	1,845,159	1,192,501
Net cash used in operating activities	(31,062,269)	(19,458,476)
Financing Activities
Deferred issuance costs	(570,218)
Proceeds from exercise of stock options	157,000
Net cash provided by financing activities	19,334,629	14,863,020
Net increase (decrease) in cash and cash equivalents	(11,727,640)	(4,595,456)
Cash and cash equivalents beginning of period	24,863,488	36,003,546
Cash and cash equivalents end of period	13,135,848	31,408,090
Series D Convertible Preferred Stock [Member]
Financing Activities
Issuance of convertible preferred stock, net of issuance costs		$ 14,863,020
Series E Convertible Preferred Stock [Member]
Financing Activities
Issuance of convertible preferred stock, net of issuance costs	$ 19,747,847

Nature of Organization and Operations	9 Months Ended
Nature of Organization and Operations	Sep. 30, 2017
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 is located 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 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 December 20, 2017, the date of issuance of these unaudited condensed consolidated financial statements, the Company believes that its cash and cash equivalents as of September 30, 2017 of $13.1 million, together with the net proceeds from its initial public offering (“IPO”) of $150.0 million and its borrowings of $20.0 million under its term loan facility and $7.0 million under a promissory note, will be sufficient to fund its operations into the first 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 biopharmaceutical companies in the clinical 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 may be unable to generate recurring product revenue or achieve profitability. The Company’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 on terms that are favorable to the Company, (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 funding, or is not able to obtain 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.

Basis of Presentation	9 Months Ended
Basis of Presentation	Sep. 30, 2017
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, 2016 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 nine months ended September 30, 2017 are not necessarily indicative of the results to be expected for the year ending December 31, 2017 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, 2016. During 2017, the Company identified and corrected an immaterial presentation error in the consolidated statements of operations and comprehensive loss for the three and nine months ended September 30, 2016. The error was the result of presenting $0.5 and $0.9 million of Australian research and development refundable credits for the three and nine months ended September 30, 2016, respectively, as an income tax benefit in those periods. As the enacted legislation in Australia provides for the credit to be received without regard to taxable income the credit was incorrectly classified as an income tax benefit. Accordingly, we have reclassified the credits as a reduction of research and development operating expenses. Net loss and comprehensive loss was not impacted by this error and no other line items within any of the other consolidated financial statements and footnotes were impacted by this error. 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. The shares of common stock retained a par value of $0.0001 per share. Accordingly, the stockholders’ deficit reflects the reverse stock split by reclassifying from “common stock” to “additional paid-in capital” in an amount equal to the par value of the decreased shares resulting from the reverse stock split. 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 will apply 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. Once effective, ASU 2014-09 will replace most of the existing revenue recognition requirements in U.S. GAAP. The FASB also issued ASU 2015-14, Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date, which deferred the effective date of the standard for one year. As a result, the new standard is effective for annual reporting periods beginning after December 15, 2017, for public companies, including interim periods within that reporting period. As the Company does not currently have revenue, the adoption of the new standard will have no impact on its consolidated financial statements. In August 2014, the FASB issued ASU 2014-15, Presentation of Financial Statements—Going Concern (Subtopic 205-40): Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern, which requires management to evaluate whether there are conditions and events that raise substantial doubt about the entity’s ability to continue as a going concern within one year after the financial statements are issued and, if so, to disclose that fact. Management is also required to evaluate and disclose whether its plans alleviate that doubt. The standard is effective for annual periods ending after December 15, 2016. With the adoption of the ASU 2014-15, The Company has expanded its disclosures within the Liquidity and Going Concern section of Note 1. In November 2015, the FASB issued ASU 2015-17, Income Taxes (Topic 740): Balance Sheet Classification of Deferred Taxes. This ASU requires the Company to classify deferred tax assets and liabilities as noncurrent amounts in the consolidated balance sheets. Such amounts were previously required to be classified as current and noncurrent assets and liabilities. The Company adopted the provisions of ASU 2015-17 effective January 1, 2017; however, there was no impact to the financial statements as a result of this adoption as the Company has recorded full valuation allowances against all deferred tax assets. 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 March 2016, the FASB issued ASU 2016-09, Compensation—Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting. The guidance simplified the accounting and financial reporting of the income tax impact of share-based compensation arrangements. This guidance requires excess tax benefits to be recorded as a discrete item within income tax expense rather than additional paid-in-capital. In addition, excess tax benefits are required to be classified as cash from operating activities rather than cash from financing activities. The Company adopted the provisions of ASU 2016-09 effective January 1, 2017. The Company elected to apply the cash flow guidance of ASU 2016-09 retrospectively to all prior periods with no impact to historical periods. The Company also adopted a change in accounting policy to recognize forfeitures of awards as they occur instead of estimating potential forfeitures.

Convertible Preferred Stock	9 Months Ended
Convertible Preferred Stock	Sep. 30, 2017
Equity [Abstract]
Convertible Preferred Stock	3. Convertible Preferred Stock Series D Convertible Preferred Stock In January 2016, the Company issued 6,714,413 shares of series D convertible preferred stock (the “Series D Preferred”) at $2.234 per share for proceeds of $15,000,000, less issuance costs of $136,980. Series E Convertible Preferred Stock In August 2017, the Company issued 7,792,035 shares of series E convertible preferred stock (the “Series E Preferred”) at $2.571 per share for aggregate proceeds of $20,033,322, less issuance costs of $285,475. Upon closing of the Company’s initial public offering on November 13, 2017, all shares of convertible preferred stock converted into shares of common stock. The rights, preferences and privileges of the convertible preferred stock were as follows: Voting Rights—All holders of convertible preferred stock and Common Stock were entitled to vote together on an as-converted basis as a single class, except as specifically set forth in the Certificate of Incorporation. So long as at least 20% of the shares of Series E Preferred issued pursuant to the Stock Purchase Agreement dated August 7, 2017 (subject to appropriate adjustments for stock splits and the like) remained outstanding, the Series E Preferred as a class were entitled to elect one member of the Board of Directors; provided that the holders of record of the shares of Series E Preferred were entitled to elect two members of the Board of Directors if the Company consummated the Second Tranche Closing as defined in the Stock Purchase Agreement. So long as 4,219,870 shares of Series D Preferred (subject to appropriate adjustments for stock splits and the like) remained outstanding, the Series D Preferred as a class were entitled to elect one member of the Board of Directors. So long as 1,920,000 shares of series C convertible preferred stock (the “Series C Preferred”) (subject to appropriate adjustments for stock splits and the like) remained outstanding, the Series C Preferred as a class were entitled to elect two members of the Board of Directors. At any time when at least 5,834,303 shares of preferred stock (subject to appropriate adjustments for stock splits and the like) remained outstanding, the Company could not without the written consent of holders of at least 60% of the then outstanding shares of preferred stock, liquidate, dissolve or wind-up the affairs of the Company, or effect any merger, sale, lease, transfer exclusive license or other disposition of all or substantially all of the assets of the Company; amend the Certificate of Incorporation or Bylaws of the Company; authorize or issue any security having rights, preferences or privileges senior to or on parity with the preferred stock; increase the authorized number of shares of Series C Preferred, Series D Preferred or Series E Preferred; reclassify, alter or amend any security pari passu with or junior to the Series C Preferred, Series D Preferred or Series E Preferred with respect to liquidation, dividends or redemption, if such reclassification, alteration or amendment would have rendered them senior to or pari passu with the Series C Preferred, Series D Preferred or Series E Preferred; pay any dividend; authorize any debt security; create or hold capital stock in any subsidiary that is not a wholly-owned subsidiary; or dispose of any subsidiary stock or all or substantially all of any subsidiary assets. Under the Voting Agreement dated August 7, 2017 (the “Voting Agreement”), the preferred stockholders agreed to vote their shares in favor of a Deemed Liquidation Event, as defined by the Certificate of Incorporation as amended to date (or other transaction in which at least a majority of the voting power of the Company is transferred), that had been approved by the Board of Directors and (a)(i) the holders of at least 60% of the outstanding shares of convertible preferred stock, or (ii) if the consideration for the Deemed Liquidation Event exceeded in $350,000,000, the holders of at least a majority of the convertible preferred stock, (b) the holders of at least a majority of the outstanding shares of Series C Preferred, (c) the holders of at least 60% of the outstanding shares of Series D Preferred and (d) the holders of at least 60% of the outstanding shares of Series E Preferred (the “Electing Parties”). Under the Voting Agreement, the Board of Directors and the Electing Parties could have required that the Company initiate a process to sell the Company. If the Board of Directors does not subsequently recommend the sale of the Company, the Board of Directors and the Electing Parties could have required that the Company subsequently initiate the process again. The Voting Agreement terminated immediately prior to the closing of the IPO. Dividends—The convertible preferred stock could not accrue any dividends. The holders of the convertible preferred stock were be entitled to participate pro rata in any dividends payable to holders of shares of common stock on an as-converted basis. Liquidation Preference—In the event of a liquidation, dissolution or winding up of the Company, the proceeds would have been paid in the following order of priority: First, the holders of Series E Preferred would have been paid the higher of (i) the Series E Issue Price on each share of Series E Preferred ($2.571), and (ii) the amount that they would be paid if they first converted their shares of Series E Preferred into common stock immediately prior to voluntary or involuntary liquidation, dissolution or winding up of the Company, or a Deemed Liquidation Event. Then, the holders of Series D Preferred would have been paid the higher of (i) the Series D Issue Price on each share of Series D Preferred ($2.234), and (ii) the amount that they would be paid if they first converted their shares of Series D Preferred into common stock immediately prior to voluntary or involuntary liquidation, dissolution or winding up of the Company, or a Deemed Liquidation Event. Next, the holders of Series C Preferred would have been paid the higher of (i) one times the Series C Issue Price on each share of Series C Preferred ($1.25), and (ii) the amount that they would be paid if they first converted their shares of Series C Preferred into common stock immediately prior to voluntary or involuntary liquidation, dissolution or winding up of the Company, or a Deemed Liquidation Event. Thereafter, the holders of Series A Preferred and Series B Preferred would have been paid the higher of (i) one times the original purchase price of each share of Series A Preferred ($1.00) or Series B Preferred ($1.10), as applicable, and (ii) the amount that they would be paid if they first converted their shares of Series A Preferred or Series B Preferred, as applicable, into common stock immediately prior to voluntary or involuntary liquidation, dissolution or winding up of the Company, or a Deemed Liquidation Event. Upon the completion of the distributions to the holders of convertible preferred stock set forth above, then the assets and funds of the Company would have been distributed ratably to the holders of common stock. Conversion—The convertible preferred stock was convertible to common stock in a ratio of 2.133:1 at any time at the option of the holder, subject to certain adjustments for stock dividends, splits, combinations and other similar events. The Series C Preferred, the Series D Preferred and the Series E Preferred would have received broad-based weighted average antidilution protection for issuances of common stock equivalents for consideration less than the Series C Issue Price, the Series D Issue Price or the Series E Issue Price, respectively. Upon closing of the Company’s IPO on November 13, 2017, all shares of convertible preferred stock converted into shares of common stock.

Share-based Compensation

9 Months Ended

Sep. 30, 2017

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Share-based Compensation

7. Share-based Compensation

The Company estimates the fair value of each stock option grant using the Monte Carlo simulation model (“Monte Carlo”) for grants made prior to June 30, 2015 and the Black-Scholes option pricing model (“Black-Scholes”) for grants made on or after July 1, 2015. Unvested awards to non-employees are re-measured at each vesting date and at each financial reporting date.

For grants in 2016 and grants in 2017 prior to the IPO, the Company’s Board of Directors sets the exercise price for stock options granted on such dates at the fair value of its common stock with the assistance of a third-party specialist. Due to the absence of a public trading market for the Company’s common stock, since inception through September 30, 2017, the Company’s retrospective and contemporaneous determinations of the fair value of its common stock were performed using methodologies, approaches and assumptions consistent with the American Institute of Certified Public Accountants Audit and Accounting Practice Aid Series: Valuation of Privately-Held-Company Equity Securities Issued as Compensation. There are significant judgments and estimates inherent in the determination of the fair value of the Company’s common stock, including the contemporaneous and retrospective valuations. These judgments and estimates include assumptions regarding the Company’s future operating performance, the time to completing an initial public offering or other liquidity event and the determinations of the appropriate valuation methods. If the Company had made different assumptions, its share-based compensation expense, net loss and net loss per share could have been significantly different.

Valuation Methodologies

The common stock valuations were prepared using a hybrid of the option-pricing method (“OPM”), and the probability-weighted expected return method (“PWERM”).

OPM. The OPM treats each class of common stock and preferred stock as call options on the total equity value of a company, with exercise prices based on the value thresholds at which the allocation among the various holders of a company’s securities changes. Under this method, the common stock has value only if the funds available for distribution to stockholders exceed the value of the preferred stock liquidation preference at the time of a liquidity event, such as a strategic sale, merger or initial public offering. The common stock is modeled as a call option on the underlying equity value at a predetermined exercise price. In the model, the exercise price is based on a comparison with the total equity value rather than, as in the case of a regular call option, a comparison with a per share stock price. Thus, common stock is considered to be a call option with a claim on the enterprise at an exercise price equal to the remaining value immediately after the preferred stock liquidation preference is paid.

The OPM uses Monte Carlo to price the call options. This model defines the securities’ fair values as functions of the current fair value of a company and uses assumptions such as the anticipated timing of a potential liquidity event and the estimated volatility of the equity securities. The aggregate value of the common stock derived from the OPM is then divided by the number of shares of common stock outstanding to arrive at the per share value.

The Company used the OPM backsolve approach to estimate enterprise value under the OPM. The OPM backsolve approach uses the OPM to calculate the implied equity value based on recent sales of the company’s securities. For the OPM, the Company based its assumed volatility factor on the historical trading volatility of its publicly traded peer companies. For each valuation date, the Company determined the appropriate volatility to be used, considering such factors as its expected time to a liquidity event and its stage of development.

To derive the fair value of the Company’s common stock using the OPM, the Company calculated the proceeds to the common stockholders based on the preferences and priorities of the preferred and common stock. The Company then applied a discount for lack of marketability to the common stock to account for the lack of access to an active public market.

PWERM. The PWERM is a scenario-based methodology that estimates the fair value of common stock based upon an analysis of future values for the company, assuming various outcomes. The common stock value is based on the probability-weighted present value of expected future investment returns considering each of the possible outcomes available as well as the rights of each class of stock. The future value of the common stock under each outcome is discounted back to the valuation date at an appropriate risk-adjusted discount rate and probability weighted to arrive at an indication of value for the common stock. A discount for lack of marketability is then applied to the common stock to account for the lack of access to an active public market.

For the Company’s contemporaneous common stock valuations as of January 22, 2016, February 8, 2016, September 16, 2016, February 8, 2017, May 18, 2017 and August 21, 2017, the Company used a hybrid of the OPM and PWERM and considered two types of future event scenarios: an initial public offering and a sale transaction. The Company valued the initial public offering scenario using the OPM backsolve approach for these valuations. The Company’s third-party valuation consultant determined the relative probability of each type of future event scenario based on an analysis of market conditions at the time, including then-current initial public offering valuations of similarly situated companies, and expectations as to the timing and likely prospects of the future-event scenarios.

To derive the fair value of the common stock for each scenario using the hybrid PWERM and OPM, the Company calculated the proceeds to the common stockholders based on the preferences and priorities of the preferred and common stock. The Company then applied a discount for lack of marketability to the common stock to account for the lack of access to an active public market.

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


	Shares			Weighted - Average Exercise Price Per Share		Weighted - Average Grant Date Fair Value Per Share		Weighted - Average Contractual Life (in years)		Aggregate Intrinsic Value
Outstanding, December 31, 2016		3,831,237		$	2.84	$	1.75		7.20	$	243,000
Granted		1,664,315			3.42		1.83		9.75		1,488,388
Exercised		(44,537	)		3.53		2.26		—		34,715
Forfeited		(298,896	)		2.44		1.14		—		558,936

Outstanding, September 30, 2017		5,152,119			3.04		1.81		7.36		6,535,348

Options exercisable, September 30, 2017		2,740,196			2.70		1.70		5.70		4,402,633

Expected to vest, September 30, 2017		2,411,923			3.43		1.95		9.24		2,132,715

Total share-based compensation expense recognized was as follows:


	Three Months Ended September 30,				Nine Months Ended September 30,
	2016		2017		2016		2017
Research and development	$	333	$	310,250	$	224,836	$	653,414
General and administrative		35,291		235,286		537,593		600,956

Total share-based compensation expense	$	35,624	$	545,536	$	762,429	$	1,254,370

Subsequent Events	9 Months Ended
Subsequent Events	Sep. 30, 2017
Subsequent Events [Abstract]
Subsequent Events	9. Subsequent Events 2010 Stock Incentive Plan In October 2017, the Company granted an aggregate of 738,392 options to purchase shares of common stock to employees with an exercise price of $10.03. Term Loan Facility and Promissory Note 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 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, an initial public offering or the expiration of the warrant, SVB may require the Company to repurchase the warrant for a total aggregate purchase price of $250,000. 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. 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 preferred stock conversion prices of the convertible preferred stock to reflect a proportional decrease in the number of shares of common stock to be issued upon conversion and in an adjustment to the number of shares of common stock underlying options to purchase common stock and warrants, as well as the related exercise prices. 2017 Stock Incentive Plan In October 2017, the Company’s 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 299,567 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. The Company granted options to purchase an aggregate of 537,737 shares of common stock under the 2017 Plan in November 2017. 2017 Employee Stock Purchase Plan In October 2017, the Company’s stockholders approved the 2017 Employee Stock Purchase Plan (the “2017 ESPP”), which became effective on November 8, 2017. Under the 2017 ESPP the Company may issue up to an aggregate of (i) 468,823 shares of Common Stock, plus (ii) an annual increase to be added on the first day of each fiscal year, beginning with the fiscal year ending December 31, 2018 and continuing for each fiscal year until, and including, the fiscal year ending December 31, 2027, equal to the least of (a) 937,646 shares of Common Stock, (b) 1% of the outstanding shares on such date and (c) an amount determined by the board of directors. No shares have been issued from the 2017 ESPP. Initial Public Offering On November 13, 2017, the Company closed its IPO in which it issued and sold 10,714,000 shares of its common stock at a price per share of $14.00. 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. Deferred issuance costs of $783,695 have been incurred and are recognized as other current assets in the September 30, 2017 condensed consolidated balance sheet, of which $570,218 had been paid, as of September 30, 2017. 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. Refundable Research and Development Credit In November 2017, the Company received a refund of $1,291,300 from the Australian tax office related to the 2016 Australian research and development credit.

Summary of Significant Accounting Policies (Policies)	9 Months Ended
Summary of Significant Accounting Policies (Policies)	Sep. 30, 2017
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, 2016 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 nine months ended September 30, 2017 are not necessarily indicative of the results to be expected for the year ending December 31, 2017 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, 2016. During 2017, the Company identified and corrected an immaterial presentation error in the consolidated statements of operations and comprehensive loss for the three and nine months ended September 30, 2016. The error was the result of presenting $0.5 and $0.9 million of Australian research and development refundable credits for the three and nine months ended September 30, 2016, respectively, as an income tax benefit in those periods. As the enacted legislation in Australia provides for the credit to be received without regard to taxable income the credit was incorrectly classified as an income tax benefit. Accordingly, we have reclassified the credits as a reduction of research and development operating expenses. Net loss and comprehensive loss was not impacted by this error and no other line items within any of the other consolidated financial statements and footnotes were impacted by this error.
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. The shares of common stock retained a par value of $0.0001 per share. Accordingly, the stockholders’ deficit reflects the reverse stock split by reclassifying from “common stock” to “additional paid-in capital” in an amount equal to the par value of the decreased shares resulting from the reverse stock split.
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 will apply 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. Once effective, ASU 2014-09 will replace most of the existing revenue recognition requirements in U.S. GAAP. The FASB also issued ASU 2015-14, Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date, which deferred the effective date of the standard for one year. As a result, the new standard is effective for annual reporting periods beginning after December 15, 2017, for public companies, including interim periods within that reporting period. As the Company does not currently have revenue, the adoption of the new standard will have no impact on its consolidated financial statements. In August 2014, the FASB issued ASU 2014-15, Presentation of Financial Statements—Going Concern (Subtopic 205-40): Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern, which requires management to evaluate whether there are conditions and events that raise substantial doubt about the entity’s ability to continue as a going concern within one year after the financial statements are issued and, if so, to disclose that fact. Management is also required to evaluate and disclose whether its plans alleviate that doubt. The standard is effective for annual periods ending after December 15, 2016. With the adoption of the ASU 2014-15, The Company has expanded its disclosures within the Liquidity and Going Concern section of Note 1. In November 2015, the FASB issued ASU 2015-17, Income Taxes (Topic 740): Balance Sheet Classification of Deferred Taxes. This ASU requires the Company to classify deferred tax assets and liabilities as noncurrent amounts in the consolidated balance sheets. Such amounts were previously required to be classified as current and noncurrent assets and liabilities. The Company adopted the provisions of ASU 2015-17 effective January 1, 2017; however, there was no impact to the financial statements as a result of this adoption as the Company has recorded full valuation allowances against all deferred tax assets. 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 March 2016, the FASB issued ASU 2016-09, Compensation—Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting. The guidance simplified the accounting and financial reporting of the income tax impact of share-based compensation arrangements. This guidance requires excess tax benefits to be recorded as a discrete item within income tax expense rather than additional paid-in-capital. In addition, excess tax benefits are required to be classified as cash from operating activities rather than cash from financing activities. The Company adopted the provisions of ASU 2016-09 effective January 1, 2017. The Company elected to apply the cash flow guidance of ASU 2016-09 retrospectively to all prior periods with no impact to historical periods. The Company also adopted a change in accounting policy to recognize forfeitures of awards as they occur instead of estimating potential forfeitures.

Share-based Compensation (Tables)

9 Months Ended

Sep. 30, 2017

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Summary of Stock Option Activity

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


	Shares			Weighted - Average Exercise Price Per Share		Weighted - Average Grant Date Fair Value Per Share		Weighted - Average Contractual Life (in years)		Aggregate Intrinsic Value
Outstanding, December 31, 2016		3,831,237		$	2.84	$	1.75		7.20	$	243,000
Granted		1,664,315			3.42		1.83		9.75		1,488,388
Exercised		(44,537	)		3.53		2.26		—		34,715
Forfeited		(298,896	)		2.44		1.14		—		558,936

Outstanding, September 30, 2017		5,152,119			3.04		1.81		7.36		6,535,348

Options exercisable, September 30, 2017		2,740,196			2.70		1.70		5.70		4,402,633

Expected to vest, September 30, 2017		2,411,923			3.43		1.95		9.24		2,132,715

Summary of Share-based Compensation Expense Recognized

Total share-based compensation expense recognized was as follows:


	Three Months Ended September 30,				Nine Months Ended September 30,
	2016		2017		2016		2017
Research and development	$	333	$	310,250	$	224,836	$	653,414
General and administrative		35,291		235,286		537,593		600,956

Total share-based compensation expense	$	35,624	$	545,536	$	762,429	$	1,254,370

Nature of Organization and Operations - Additional Information (Detail) - USD ($)		9 Months Ended
	Dec. 20, 2017	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Date of incorporation		2009-09
Cash and cash equivalents		$ 13,135,848	$ 24,863,488	$ 31,408,090	$ 36,003,546
Proceeds from Initial public offering	$ 150,000,000
Term loan	20,000,000
Promissory note	$ 7,000,000

Convertible Preferred Stock - Additional Information (Detail)		1 Months Ended		9 Months Ended
	Aug. 07, 2017 USD ($) shares	Aug. 31, 2017 USD ($) $ / shares shares	Jan. 31, 2016 USD ($) $ / shares shares	Sep. 30, 2017 USD ($) $ / shares shares	Sep. 30, 2016 USD ($)	Dec. 31, 2016 $ / shares shares
Class of Stock [Line Items]
Convertible preferred stock issuance costs \| $				$ 570,218
Preferred shares outstanding \| shares	5,834,303
Conversion ratio		2.1331
Preferred Stock [Member]
Class of Stock [Line Items]
Percentage of shares outstanding	60.00%
Series D Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Issuance of convertible preferred stock \| shares			6,714,413	21,099,351		21,099,351
Convertible preferred stock issued, price per share \| $ / shares			$ 2.234	$ 0.0001		$ 0.0001
Net proceeds from issuance and sale of convertible preferred stock \| $			$ 15,000,000		$ 14,863,020
Convertible preferred stock issuance costs \| $			$ 136,980
Preferred shares outstanding \| shares				21,099,351		21,099,351
Series E Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Issuance of convertible preferred stock \| shares		7,792,035		7,792,035
Convertible preferred stock issued, price per share \| $ / shares		$ 2.571		$ 0.0001		$ 0.0001
Net proceeds from issuance and sale of convertible preferred stock \| $				$ 19,747,847
Convertible preferred stock issuance costs \| $		$ 285,475
Gross proceeds from issuance and sale of convertible preferred stock \| $		$ 20,033,322
Preferred shares outstanding \| shares				7,792,035
Series E Preferred Stock [Member]
Class of Stock [Line Items]
Percentage of shares outstanding	60.00%
Liquidation preference price per share \| $ / shares		$ 2.571
Series E Preferred Stock [Member] \| Stock Purchase Agreement [Member]
Class of Stock [Line Items]
Percentage of shares outstanding	20.00%
Series D Preferred Stock [Member]
Class of Stock [Line Items]
Percentage of shares outstanding	60.00%
Preferred shares outstanding \| shares	4,219,870
Liquidation preference price per share \| $ / shares		2.234
Series C Preferred Stock [Member]
Class of Stock [Line Items]
Preferred shares outstanding \| shares	1,920,000
Liquidation preference price per share \| $ / shares		1.25
Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Percentage of shares outstanding	60.00%
Convertible Preferred Stock [Member] \| Minimum [Member]
Class of Stock [Line Items]
Consideration for Deemed Liquidation Event \| $	$ 350,000,000
Series A Preferred Stock [Member]
Class of Stock [Line Items]
Liquidation preference price per share \| $ / shares		1.00
Series B Preferred Stock [Member]
Class of Stock [Line Items]
Liquidation preference price per share \| $ / shares		$ 1.10

Refundable Research and Development Credit and Income Taxes - Additional Information (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current income tax expense or benefit	$ 0	$ 0
Deferred income tax expense or benefit	0	$ 0
Deferred tax asset net operating loss carryforwards	38,895,141
Research and development tax credit carryforwards	$ 7,913,409

Share-based Compensation - Summary of Stock Option Activity (Detail) - USD ($)		9 Months Ended
	Dec. 31, 2016	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Shares Outstanding, Beginning balance		3,831,237
Shares, Granted		1,664,315
Shares, Exercised		(44,537)
Shares, Forfeited		(298,896)
Shares Outstanding, Ending balance	3,831,237	5,152,119
Shares, Options exercisable, September 30, 2017		2,740,196
Shares, Expected to vest, September 30, 2017		2,411,923
Weighted -Average Exercise Price Per Share Outstanding, Beginning balance		$ 2.84
Weighted -Average Exercise Price Per Share, Granted		3.42
Weighted -Average Exercise Price Per Share, Exercised		3.53
Weighted -Average Exercise Price Per Share, Forfeited		2.44
Weighted -Average Exercise Price Per Share, Outstanding, Ending balance	$ 2.84	3.04
Weighted -Average Exercise Price Per Share, Options exercisable, September 30, 2017		2.70
Weighted -Average Exercise Price Per Share, Expected to vest, September 30, 2017		3.43
Weighted - Average Grant Date Fair Value Per Share Outstanding, Beginning balance		1.75
Weighted - Average Grant Date Fair Value Per Share, Granted		1.83
Weighted - Average Grant Date Fair Value Per Share, Exercised		2.26
Weighted - Average Grant Date Fair Value Per Share, Forfeited		1.14
Weighted - Average Grant Date Fair Value Per Share Outstanding, Ending balance	$ 1.75	1.81
Weighted - Average Grant Date Fair Value Per Share, Options exercisable, September 30, 2017		1.70
Weighted - Average Grant Date Fair Value Per Share, Expected to vest, September 30, 2017		$ 1.95
Weighted - Average Contractual Life, Granted		9 years 9 months
Weighted - Average Contractual Life Outstanding, Balance	7 years 2 months 12 days	7 years 4 months 9 days
Weighted - Average Contractual Life, Options exercisable, September 30, 2017		5 years 8 months 12 days
Weighted - Average Contractual Life, Expected to vest, September 30, 2017		9 years 2 months 27 days
Aggregate Intrinsic Value Outstanding, Beginning balance		$ 243,000
Aggregate Intrinsic Value, Granted		1,488,388
Aggregate Intrinsic Value, Exercised		34,715
Aggregate Intrinsic Value, Forfeited		558,936
Aggregate Intrinsic Value, Outstanding, Ending balance	$ 243,000	6,535,348
Aggregate Intrinsic Value, Options exercisable, September 30, 2017		4,402,633
Aggregate Intrinsic Value, Expected to vest, September 30, 2017		$ 2,132,715

Subsequent Events - Term Loan Facility and Promissory Note - Additional Information (Detail)			9 Months Ended
	Oct. 20, 2017 USD ($) Installment $ / shares shares	Oct. 19, 2017 USD ($) $ / shares shares	Sep. 30, 2017	Dec. 20, 2017 USD ($)
Subsequent Event [Line Items]
Term loan facility				$ 20,000,000
Unsecured Promissory Note [Member] \| Golda Darty Partners S A [Member]
Subsequent Event [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.
Loan and Security Agreement [Member] \| Silicon Valley Bank [Member]
Subsequent Event [Line Items]
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.
Subsequent Event [Member] \| Term Loan Facility [Member]
Subsequent Event [Line Items]
Final payment due amount	$ 1,600,000
Subsequent Event [Member] \| Unsecured Promissory Note [Member] \| Golda Darty Partners S A [Member]
Subsequent Event [Line Items]
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
Subsequent Event [Member] \| Minimum [Member] \| Term Loan Facility [Member]
Subsequent Event [Line Items]
Prepayment fee, percentage	0.00%
Subsequent Event [Member] \| Maximum [Member] \| Term Loan Facility [Member]
Subsequent Event [Line Items]
Prepayment fee, percentage	2.00%
Subsequent Event [Member] \| Silicon Valley Bank [Member] \| Term Loan Facility [Member]
Subsequent Event [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
Subsequent Event [Member] \| Loan and Security Agreement [Member] \| Silicon Valley Bank [Member]
Subsequent Event [Line Items]
Term loan facility	$ 20,000,000
Increase in interest rate	5.00%
Subsequent Event [Member] \| Loan and Security Agreement [Member] \| Silicon Valley Bank [Member] \| Prime Rate [Member]
Subsequent Event [Line Items]
Interest bear rate	1.50%
Subsequent Event [Member] \| Loan and Security Agreement [Member] \| Silicon Valley Bank [Member] \| Convertible Preferred Stock [Member]
Subsequent Event [Line Items]
Debt instrument, number of equal monthly installments \| Installment	24

Subsequent Events - 2017 Stock Incentive Plan - Additional Information (Detail) - shares	1 Months Ended	9 Months Ended
	Oct. 31, 2017	Sep. 30, 2017
Subsequent Event [Line Items]
Stock incentive plan, number of shares granted		1,664,315
2017 Stock Incentive Plan [Member]
Subsequent Event [Line Items]
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.
Subsequent Event [Member] \| 2017 Stock Incentive Plan [Member]
Subsequent Event [Line Items]
Common stock shares reserved for issuance	1,359,587
Number of shares available for future grant	299,567
Shares issuable increased during the period	4,219,409
Percentage of shares issued on common stock outstanding	4.00%
Stock incentive plan, number of shares granted	537,737

Subsequent Events - 2017 Employee Stock Purchase Plan - Additional Information (Detail) - 2017 Employee Stock Purchase Plan [Member] - shares	1 Months Ended	9 Months Ended
	Oct. 31, 2017	Sep. 30, 2017
Subsequent Event [Line Items]
Common Stock Issuance Description		Equal to the least of (a) 937,646 shares of Common Stock, (b) 1% of the outstanding shares on such date and (c) an amount determined by the board of directors.
Subsequent Event [Member]
Subsequent Event [Line Items]
Common stock shares reserved for issuance	468,823
Shares issuable increased during the period	937,646
Percentage of shares issued on common stock outstanding	1.00%
Stock incentive plan, number of shares issued	0

Subsequent Events - Initial Public Offering - Additional Information (Detail) - USD ($)				9 Months Ended
	Dec. 20, 2017	Dec. 13, 2017	Nov. 13, 2017	Sep. 30, 2017
Subsequent Event [Line Items]
Proceeds from initial offering after deducting underwriting discounts and commissions	$ 150,000,000
Deferred issuance costs				$ 570,218
Subsequent Event [Member] \| IPO [Member]
Subsequent Event [Line Items]
Number of additional shares issued and sold		981,107	10,714,000
Public offering price of common stock		$ 14.00	$ 14.00
Proceeds from initial offering after deducting underwriting discounts and commissions		$ 12,800,000	$ 137,200,000
Underwriting discounts and commissions		$ 1,000,000	10,500,000
Initial public offering costs			2,300,000
Deferred issuance costs			783,695
Deferred issuance costs			$ 570,218

Commitments and Contingencies	9 Months Ended
Commitments and Contingencies	Sep. 30, 2017
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	6. Commitments and Contingencies In April 2017, the Company entered into a lease for office space in Waltham, Massachusetts (the “Waltham Lease”). The Waltham Lease has a term of 60 months and includes leasing 6,126 square feet of office space. The Waltham Lease provides for initial monthly lease payments of $17,612 per month. The base rent payable over the lease period is $1,117,995. In May 2017, the Company entered into a lease for office space in San Francisco, California (the “SF Lease”). The SF Lease has a term of 24 months and includes leasing 1,872 square feet of office space. The SF Lease provides for initial monthly lease payments of $7,800 per month. The base rent payable over the lease period is $182,208. The Company contracts to conduct research and development activities with remaining contract costs of approximately $1,339,000 at September 30, 2017, 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.

Basis of Presentation - Additional Information (Detail)		3 Months Ended	9 Months Ended
Basis of Presentation - Additional Information (Detail)	Oct. 27, 2017 $ / shares	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) $ / shares	Sep. 30, 2016 USD ($)	Dec. 31, 2016 $ / shares
Summary Of Significant Accounting Policies [Line Items]
Refundable research and development credit \| $		$ 500,000	$ (895,761)	$ (953,751)
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, par value			$ 0.0001		$ 0.0001
Subsequent Event [Member]
Summary Of Significant Accounting Policies [Line Items]
Common stock reverse stock split, conversion ratio	0.468823254
Common stock, par value	$ 0.0001

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($)	Sep. 30, 2017	Dec. 31, 2016
Accrued Liabilities, Current [Abstract]
Accrued research and development	$ 1,660,950	$ 871,066
Accrued payroll liabilities	1,072,751	91,598
Other	203,184	129,062
Total	$ 2,936,885	$ 1,091,726

Commitments and contingencies - Additional Information (Detail)	1 Months Ended
	May 31, 2017 USD ($) ft²	Apr. 30, 2017 USD ($) ft²	Sep. 30, 2017 USD ($)
Commitments And Contingencies [Line Items]
Remaining contract costs			$ 1,339,000
Waltham, Massachusetts [Member]
Commitments And Contingencies [Line Items]
Office space lease term		60 months
Area of operating lease \| ft²		6,126
Initial monthly lease payments per month		$ 17,612
Base rent payable		$ 1,117,995
San Francisco, California [Member]
Commitments And Contingencies [Line Items]
Office space lease term	24 months
Area of operating lease \| ft²	1,872
Initial monthly lease payments per month	$ 7,800
Base rent payable	$ 182,208

Net Loss per Share - Summary of Shares Outstanding that were Excluded from Calculation of Diluted Net Loss Per Share (Detail) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount	35,222,153	30,248,133	35,222,153	30,248,133
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	30,070,034	26,416,896	30,070,034	26,416,896
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	5,152,119	3,831,237	5,152,119	3,831,237

Refundable Research and Development Credit and Income Taxes	9 Months Ended
	Sep. 30, 2017
Text Block [Abstract]
Refundable Research and Development Credit and Income Taxes	5. 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 nine months ended September 30, 2016 and 2017, there was no current or deferred income tax expense or benefit due to the Company’s net losses and increases in its deferred tax asset valuation allowance during those periods. The deferred tax asset includes approximately $38,895,141 of tax affected net operating loss carryforwards and $7,913,409 of research and development tax credit carryforwards. The Company has recorded a full valuation allowance against these deferred tax assets which are also subject to further review for full recoverability.

Subsequent Events - Reverse Stock Split - Additional Information (Detail)		9 Months Ended
	Oct. 27, 2017	Sep. 30, 2017
Subsequent Event [Line Items]
Reverse stock split, description		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.
Subsequent Event [Member]
Subsequent Event [Line Items]
Reverse stock split, ratio	0.468823254
Reverse stock split, effective date	Oct. 27, 2017

Subsequent Events - Automatic Conversion of Preferred Stock - Additional Information (Detail)	Nov. 13, 2017 shares
Subsequent Event [Member]
Subsequent Event [Line Items]
Conversion of preferred stock to common stock	30,070,034

Subsequent Events - Refundable Research and Development Credit - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended	9 Months Ended
	Nov. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Subsequent Event [Line Items]
Proceeds from refund of research and development credit		$ 500,000	$ (895,761)	$ (953,751)
Australian Tax Office [Member]
Subsequent Event [Line Items]
Proceeds from refund of research and development credit	$ 1,291,300