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Document and Entity Information	12 Months Ended
Document and Entity Information	Dec. 31, 2017
Document And Entity Information [Abstract]
Document Type	S-1
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Trading Symbol	APLS
Entity Registrant Name	Apellis Pharmaceuticals, Inc.
Entity Central Index Key	0001492422
Entity Filer Category	Non-accelerated Filer

Consolidated Balance Sheets - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 175,643,529	$ 24,863,488
Refundable research and development credit	1,297,361	1,347,804
Prepaid assets	5,059,593	1,132,438
Other current assets	14,823	25,000
Total current assets	182,015,306	27,368,730
Other assets	116,150	64,528
Total assets	182,131,456	27,433,258
Current liabilities:
Accounts payable	3,663,253	2,547,212
Accrued expenses	2,890,705	1,091,726
Total current liabilities	6,553,958	3,638,938
Long-term liabilities:
Term loan facility	19,806,944
Promissory note - related party	6,583,402
Common stock warrant liability	244,292
Total liabilities	33,188,596	3,638,938
Stockholders' equity:
Preferred stock value
Common stock, $0.0001 par value; 87,000,000 shares authorized and 8,428,366 shares issued and outstanding at December 31, 2016 and 200,000,000 shares authorized and 50,334,152 shares issued and outstanding at December 31, 2017	5,033	843
Additional paid in capital	298,201,480	29,996,110
Accumulated deficit	(149,263,653)	(98,257,559)
Total stockholders' equity	148,942,860	23,794,320
Total liabilities and stockholders' equity	$ 182,131,456	27,433,258
Series A Convertible Preferred Stock [Member]
Stockholders' equity:
Preferred stock value		2,654,405
Series B Convertible Preferred Stock [Member]
Stockholders' equity:
Preferred stock value		6,944,148
Series C Convertible Preferred Stock [Member]
Stockholders' equity:
Preferred stock value		35,542,707
Series D Convertible Preferred Stock [Member]
Stockholders' equity:
Preferred stock value		$ 46,913,666

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	0
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	87,000,000
Common stock, issued	50,334,152	8,428,366
Common stock, outstanding	50,334,152	8,428,366
Series A Convertible Preferred Stock [Member]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	0	2,670,000
Preferred stock, issued	0	2,670,000
Preferred stock, outstanding	0	2,670,000
Series B Convertible Preferred Stock [Member]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	0	6,362,658
Preferred stock, issued	0	6,362,658
Preferred stock, outstanding	0	6,362,658
Series C Convertible Preferred Stock [Member]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	0	26,215,411
Preferred stock, issued	0	26,215,411
Preferred stock, outstanding	0	26,215,411
Series D Convertible Preferred Stock [Member]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	0	21,099,351
Preferred stock, issued	0	21,099,351
Preferred stock, outstanding	0	21,099,351

Consolidated Statements of Operations and Comprehensive Loss - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating expenses:
Research and development	$ 40,303,878	$ 22,978,599	$ 13,730,311
Cost of acquired in-process research and development			26,486,000
General and administrative	10,463,151	4,303,743	6,356,782
Operating loss	(50,767,029)	(27,282,342)	(46,573,093)
Loss from remeasurement of fair value of warrants	(153,692)
Interest income (expense), net	(96,915)	135,309	50,853
Other income, net	11,542	22,396	6,284
Net loss and comprehensive loss	$ (51,006,094)	$ (27,124,637)	$ (46,515,956)
Net loss per common share, basic and diluted	$ (3.68)	$ (3.22)	$ (8.03)
Weighted-average number of common shares used in net loss per common share, basic and diluted	13,870,949	8,428,366	5,795,040

Consolidated Statements of Changes in Stockholder's Equity - USD ($)	Total	IPO [Member]	Series E Convertible Preferred Stock [Member]	Series C Convertible Preferred Stock [Member]	Series D Convertible Preferred Stock [Member]	Convertible Preferred Stock [Member]	Common Stock [Member]	Common Stock [Member] IPO [Member]	Additional Paid-In Capital [Member]	Additional Paid-In Capital [Member] IPO [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2014	$ 13,224,707					$ 35,866,260	$ 458		$ 1,974,955		$ (24,616,966)
Beginning balance, Shares at Dec. 31, 2014						29,064,736	4,583,282
Issuance of common stock upon exercise of stock options	$ 1,719						$ 1		1,718
Issuance of common stock upon exercise of stock options, shares	732						732
Issuance of stock, net of issuance costs				$ 9,277,112	$ 32,050,646
Issuance of stock, net of issuance costs, shares				6,183,333	14,384,938
Series C tranche Right				$ (2,112)
Issuance of common stock upon closing Potentia asset purchase, net of issuance costs	$ 26,395,132						$ 384		26,394,748
Issuance of common stock upon closing Potentia asset purchase, net of issuance costs, shares							3,844,352
Share-based compensation expense	545,701								545,701
Net loss	(46,515,956)										(46,515,956)
Ending balance at Dec. 31, 2015	34,976,949					$ 77,191,906	$ 843		28,917,122		(71,132,922)
Ending balance, Shares at Dec. 31, 2015						49,633,007	8,428,366
Issuance of stock, net of issuance costs					$ 14,863,020
Issuance of stock, net of issuance costs, shares					6,714,413
Share-based compensation expense	1,078,988								1,078,988
Net loss	(27,124,637)										(27,124,637)
Ending balance at Dec. 31, 2016	23,794,320					$ 92,054,926	$ 843		29,996,110		(98,257,559)
Ending balance, Shares at Dec. 31, 2016						56,347,420	8,428,366
Issuance of common stock upon exercise of stock options	$ 164,501						$ 5		164,496
Issuance of common stock upon exercise of stock options, shares	46,881						46,881
Issuance of stock, net of issuance costs			$ 19,747,847
Issuance of stock, net of issuance costs, shares			7,792,035
Issuance of warrants to an affiliate of stockholder	$ 430,160								430,160
Issuance of common stock upon exercise of warrants	514,200						$ 9		514,191
Issuance of common stock upon exercise of warrants, shares							93,764
Conversion of convertible preferred stock to common stock						$ (111,802,773)	$ 3,007		111,799,766
Conversion of convertible preferred stock to common stock, shares						(64,139,455)	30,070,034
Issuance of common stock in initial public offering, net of issuance costs		$ 149,879,216						$ 1,169		$ 149,878,047
Issuance of common stock in initial public offering, net of issuance costs, shares								11,695,107
Share-based compensation expense	5,418,710								5,418,710
Net loss	(51,006,094)										(51,006,094)
Ending balance at Dec. 31, 2017	$ 148,942,860						$ 5,033		$ 298,201,480		$ (149,263,653)
Ending balance, Shares at Dec. 31, 2017							50,334,152

Condensed Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Activities
Net loss	$ (51,006,094)	$ (27,124,637)	$ (46,515,956)
Adjustments to reconcile net loss to net cash used in operating activities:
Non-cash acquisition of in-process research and development			26,486,000
Share-based compensation expense	5,418,710	1,078,988	545,701
Loss from remeasurement of fair value of warrants	153,692
Accretion of discounts	13,563
Amortization	11,935
Changes in operating assets and liabilities:
Refundable research and development credit	50,443	408,934	(1,313,398)
Prepaid assets	(3,927,155)	(873,884)	(190,926)
Other current assets	10,177		110,054
Other assets	(51,622)	68,743	(96,050)
Accounts payable	932,299	69,779	1,725,904
Accrued expenses	1,798,979	368,999	392,724
Net cash used in operating activities	(46,595,073)	(26,003,078)	(18,855,947)
Financing Activities
Deferred issuance costs			(90,868)
Proceeds from issuance of term loan, net of issuance costs	19,795,007
Proceeds from issuance of promissory note payable to an affiliate of stockholder	6,569,840
Proceeds from issuance of common stock, net of issuance costs	150,062,959		1,719
Proceeds from the issuance of common stock warrants	520,760
Proceeds from exercise of stock options and warrants	678,701
Net cash provided by financing activities	197,375,114	14,863,020	41,236,498
Net increase (decrease) in cash and cash equivalents	150,780,041	(11,140,058)	22,380,551
Cash and cash equivalents at beginning of period	24,863,488	36,003,546	13,622,995
Cash and cash equivalents at end of period	175,643,529	24,863,488	36,003,546
Supplemental Disclosure of Non-Cash Financing Activities
Conversion of convertible preferred stock to common stock	111,802,773
Series D Convertible Preferred Stock [Member]
Financing Activities
Issuance of convertible preferred stock, net of issuance costs		$ 14,863,020	32,050,646
Series C Convertible Preferred Stock [Member]
Financing Activities
Issuance of convertible preferred stock, net of issuance costs			$ 9,275,001
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	12 Months Ended
Nature of Organization and Operations	Dec. 31, 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. The Company’s principal executive offices are 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”). 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 to the public of $14.00 in its initial public offering (“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 deducting underwriting discounts and commissions of $1.0 million. Liquidity and Going Concern The accompanying consolidated financial statements have been prepared on a going concern basis, which contemplates the realization of assets and the satisfaction of liabilities and commitments in the normal course of business. 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. The 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, raise substantial doubt about the Company’s ability to continue as a going concern. The consolidated financial statements do not include any adjustments relating to the recoverability and classification of recorded asset amounts or amounts of liabilities that might be necessary should the Company be unable to continue its operations. If the Company is not able to obtain the required funding in the near future, or is not able to obtain funding on terms that are favorable to the Company, it will have a material adverse effect on its operations and strategic development plan for future growth. If the Company cannot successfully raise additional funding and implement its strategic development plan, then its liquidity, financial condition and business prospects will be materially and adversely affected, and the Company may have to cease operations.

Summary of Significant Accounting Policies	12 Months Ended
Summary of Significant Accounting Policies	Dec. 31, 2017
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The accompanying 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 consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and following the requirements of the Securities and Exchange Commission (the “SEC”). Reclassifications Certain prior year amounts have been reclassified to conform to the 2017 presentation. 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 the Company’s then outstanding series of convertible preferred stock to reflect a proportional decrease in the number of shares of common stock to be issued upon conversion. The accompanying 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” an amount equal to the par value of the decreased shares resulting from the reverse stock split. Offering Costs Offering costs represent underwriting, legal, accounting and other direct costs related to the Company’s IPO. These costs were deferred until completion of the IPO, at which time they were reclassified to additional paid-in capital as a reduction of the proceeds. Segment Information Operating segments are defined as components of an enterprise about which separate discrete information is available for evaluation by the chief operating decision maker, or decision-making group, in deciding how to allocate resources and in assessing performance. The Company views its operations and manages its business in one operating segment. Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results may differ from those estimates. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates, and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes, and management must select an amount that falls within that range of reasonable estimates. Estimates are used in the following areas, among others: share-based compensation expense, fair value of common stock and preferred stock, accrued expenses, prepaid expenses and income taxes. Historically for all periods prior to the IPO, the Company utilized various valuation methodologies in accordance with the framework of the American Institute of Certified Public Accountants Technical Practice Aid, Valuation of Privately-Held Company Equity Securities Issued as Compensation, to estimate the retrospective fair value of its common stock during all periods presented. The methodologies include a probability analysis including both a potential public trading scenario and potential sale scenario. In both scenarios, value is estimated using the guideline public company method. The sale scenario includes an adjustment for a market participant acquisition premium. Value is allocated among the preferred and common shares according to the rights associated with each type of security. Valuation methodologies include estimates and assumptions that require the Company’s judgment. These estimates include assumptions regarding future performance. Significant changes to the key assumptions used in the valuations could result in different fair values of common stock and the associated fair value of stock options granted at each valuation date. Since the completion of the IPO, the Company’s board of directors determines the fair value of underlying common stock based upon the closing price of the Company’s common stock as reported on the date of grant. 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. Cash and Cash Equivalents Cash and cash equivalents are defined as cash in banks and investment instruments having maturities of three months or less from their acquisition date. The carrying amounts reported in the consolidated balance sheets for cash and cash equivalents are valued at cost, which approximates their fair value. Foreign Currency The functional currency of the Company’s wholly-owned subsidiary is the U.S. dollar. Research and Development Costs incurred in connection with research and development activities are expensed as incurred. Research and development expenses include (i) employee-related expenses, including salaries, benefits, travel and share-based compensation expense; (ii) external research and development expenses incurred under arrangements with third parties, such as contract research and contract manufacturing organizations, investigational sites and consultants, including share-based compensation expense for consultants; (iii) the cost of acquiring, developing and manufacturing clinical study materials; and (iv) costs associated with preclinical and clinical activities and regulatory operations. The Company enters into consulting, research and other agreements with commercial entities, researchers, universities and others for the provision of goods and services. Such arrangements are generally cancellable upon reasonable notice and payment of costs incurred. Costs are considered incurred based on an evaluation of the progress to completion of specific tasks under each contract using information and data provided by the Company’s clinical sites and vendors. These costs consist of direct and indirect costs associated with specific projects, as well as fees paid to various entities that perform certain research on behalf of the Company. Depending upon the timing of payments to the service providers, the Company recognizes prepaid expenses or accrued expenses related to these costs. These accrued or prepaid expenses are based on management’s estimates of the work performed under service agreements, milestones achieved and experience with similar contracts. The Company monitors each of these factors and adjusts estimates accordingly. Included in research and development is a refundable Australian research and development credit. The credit is recognized as a reduction of clinical trial costs over the periods necessary to match the benefit of the credit with the costs for which it is intended to compensate. During 2017, the Company identified and corrected an immaterial presentation error in the consolidated statements of operations and comprehensive loss for the years ended December 31, 2015 and 2016. The error was the result of presenting $1.7 and $1.2 million of Australian research and development refundable credits for the years ended December 31, 2015 and 2016, respectively as an income tax benefit in the period. 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, other than the Company’s tax footnote. Patents Costs incurred in connection with the application for and issuance of patents are expensed as incurred. Income Taxes Income taxes are recorded in accordance with Financial Accounting Standards Board (“FASB”) ASC Topic 740, Income Taxes (“ASC 740”), which provides for deferred taxes using an asset and liability approach. The Company recognizes deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements or tax returns. Deferred tax assets and liabilities are determined based on the difference between the financial statement and tax bases of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Valuation allowances are provided if, based upon the weight of available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized. The Company accounts for uncertain tax positions in accordance with the provisions of ASC 740. When uncertain tax positions exist, the Company recognizes the tax benefit of tax positions to the extent that the benefit will more likely than not be realized. The determination as to whether the tax benefit will more likely than not be realized is based upon the technical merits of the tax position, as well as consideration of the available facts and circumstances. As of December 31, 2016 and 2017, the Company did not have any significant uncertain tax positions. Share-based Compensation The Company accounts for its share-based compensation awards in accordance with FASB ASC Topic 718, Compensation—Stock Compensation (“ASC 718”). ASC 718 requires all share-based payments to employees, including grants of employee stock options, be recognized in the statements of operations and comprehensive loss based on their estimated fair values over the requisite service periods for each award. The Company accounts for share-based awards to non-employees in accordance with FASB ASC Topic 505-50, Equity-Based Payments to Non-Employees (“ASC 505-50”), which requires the fair value of the award to be re-measured at fair value until a performance commitment is reached or counterparty performance is complete. The Company’s share-based compensation awards are comprised of stock options. 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 through October 2017. After the Company’s IPO in November 2017, the Company’s board of directors determined the fair value of underlying common stock based upon the closing price of the Company’s common stock as reported on the date of the grant. Both Monte Carlo and Black-Scholes for option pricing require the input of the six minimum considerations detailed in ASC 718, including (a) the expected stock price volatility, (b) the calculation of expected term of the award, (c) the risk-free interest rate and (d) expected dividends. Due to the lack of a public market for the trading of the Company’s common stock and a lack of company-specific historical and implied volatility data, the Company has based its estimate of expected volatility on the historical volatility of a group of comparable companies that are publicly traded. The historical volatility is calculated based on a period of time commensurate with the expected term assumption. The computation of expected volatility is based on the historical volatility of a representative group of companies with similar characteristics to the Company, including stage of product development and life science industry focus. The Company is in a very early stage of product development with no revenues, and the representative group of companies has similar characteristics. The Company believes the group selected has sufficient similar economic and industry characteristics. The Company calculated the expected term for options granted to employees based on a quarterly weighted average probability of exit analysis considering milestones that the Company had achieved and each of the potential exit scenarios available to the Company at that time. The expected term is applied to the stock option grant group as a whole, as the Company does not expect substantially different exercise or post-vesting termination behavior among its employee population. For options granted to non-employees, the Company utilizes the contractual term of the arrangement as the basis for the expected term assumption. The risk-free interest rate is based on a treasury instrument whose term is consistent with the expected life of the stock options. The expected dividend yield is assumed to be zero as the Company has never paid dividends and has no current plans to pay any dividends on its common stock. The Company’s share-based awards are subject to service based vesting conditions. Compensation expense related to awards to employees with service based vesting conditions is recognized on a straight-line basis based on the grant date fair value over the associated service period of the award, which is generally the vesting term. Consistent with the guidance in ASC 505-50, compensation expense related to awards to non-employees with service based vesting conditions is recognized on a straight-line basis based on the then-current fair value at each financial reporting date prior to the measurement date over the associated service period of the award, which is generally the vesting term. The Company recognizes forfeitures for employee and non-employee grants at the time they occur. Share-based compensation expense recognized in the financial statements is based on awards that are ultimately expected to vest. Concentrations of Credit Risk Cash and accounts receivable are the only financial instruments that potentially subject the Company to concentrations of credit risk. The Company’s cash is maintained with accredited financial institutions and is insured by the Federal Deposit Insurance Corporation. Net Loss per Share Basic net loss per common share is calculated by dividing net loss by the weighted-average shares outstanding during the period. For purposes of the diluted net loss per share calculation, convertible preferred stock and common stock options are considered to be common stock equivalents but have been excluded from the calculation of diluted net loss per share, as their effect would be anti-dilutive for all periods presented. Therefore, basic and diluted net loss per share were the same for all periods presented. Comprehensive Loss The Company has no components of comprehensive loss other than its net loss and, accordingly, comprehensive loss is the same as the net loss for all periods presented. Recent Accounting Pronouncements In May 2014, the 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. 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: Balance Sheet Classification of Deferred Taxes. This ASU simplifies the presentation of deferred income taxes and requires that deferred tax liabilities and assets be classified 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 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 for 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.

Asset Purchase From Related Party	12 Months Ended
Asset Purchase From Related Party	Dec. 31, 2017
Related Party Transactions [Abstract]
Asset Purchase From Related Party	3. Asset Purchase From Related Party In September 2014, the Company agreed to purchase substantially all of the assets of Potentia Pharmaceuticals, Inc. (“Potentia”), in exchange for 3,844,352 shares of common stock. In September 2015, the Company completed the acquisition and issued the shares, which the Company determined, with the assistance of a third-party specialist, to have a fair value of $26,486,000. As there were no integrated sets of activities and assets representing inputs and processes that would constitute a business under ASC 805, Business Combinations (“ASC 805”), the Company accounted for the transaction as an asset acquisition. In-process research and development expense of $26,486,000 was recognized in the Company’s consolidated statement of operations and comprehensive loss in 2015. Certain officers and members of the Board of Directors of the Company are directors and significant stockholders of Potentia. Under the asset purchase agreement, the Company agreed to make certain vendor payments on Potentia’s behalf pending the closing of the transaction. The Company recognized expenses related to this arrangement of $874,427 for the year ended December 31, 2015.

Common Stock

12 Months Ended

Dec. 31, 2017

Equity [Abstract]

Common Stock

4. Common Stock

The Company has reserved the following shares of common stock for future issuance:

	December 31,
	2015		2016		2017
Conversion of Series A convertible preferred stock		1,251,758		1,251,758		—
Conversion of Series B convertible preferred stock		2,982,962		2,982,962		—
Conversion of Series C convertible preferred stock		12,290,394		12,290,394		—
Conversion of Series D convertible preferred stock		6,743,993		9,891,866		—
Shares reserved under 2010 Equity Incentive Plan		3,375,527		4,781,264		5,817,844
Shares reserved under 2017 Equity Incentive Plan		—		—		1,682,596
Shares reserved under 2017 Employee Stock Purchase Plan						468,823
Common stock warrants		—		—		14,064
Total		26,644,634		31,198,244		7,983,327

Convertible Preferred Stock	12 Months Ended
Convertible Preferred Stock	Dec. 31, 2017
Equity [Abstract]
Convertible Preferred Stock	5. Convertible Preferred Stock Authorized capital stock includes 56,347,420 shares of convertible preferred stock and 10,000,000 shares of preferred stock at December 31, 2016 and 2017, respectively. Series C Convertible Preferred Stock In January 2015, the Company issued 183,333 shares of Series C convertible preferred stock (“Series C Preferred”) at $1.50 per share for aggregate proceeds of $275,000. Between March and May 2015, the Company issued 6,000,000 shares of Series C Preferred at $1.50 per share for aggregate proceeds of $9,000,000. Series D Convertible Preferred Stock In December 2015, the Company issued 14,384,938 shares of Series D convertible preferred stock (“Series D Preferred”) at $2.234 per share for aggregate proceeds of $32,135,951, less issuance costs of $85,306. In January 2016, the Company issued 6,714,413 shares of Series D convertible preferred stock at $2.234 per share for aggregate 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 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

12 Months Ended

Dec. 31, 2017

Text Block [Abstract]

Accrued Expenses Text Block

6. Accrued Expenses

Accrued expenses are as follows:

	December 31,
	2016		2017
Accrued research and development	$	871,066	$	2,463,808
Accrued payroll liabilities		91,598		251,491
Other		129,062		175,406
Total	$	1,091,726	$	2,890,705

Long-term Debt

12 Months Ended

Dec. 31, 2017

Debt Disclosure [Abstract]

Long-term Debt

7. 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, thereafter from November 1, 2019 through October 1, 2021, at which time 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 University of Pennsylvania (“Penn”) (described in Note 8) 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. As the warrant may be put to the Company at SVB’s option based on the passage of time (upon the expiration of the warrant), the Company has accounted for the warrant as a liability pursuant to ASC 480.

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 December 31, 2017 are as follows:

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

License Agreements	12 Months Ended
License Agreements	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
License Agreements	8. License Agreements In connection with its purchase of assets from Potentia in September 2014, the Company became party to a license agreement with the Trustees of the University of Pennsylvania (“Penn”) as a result of an agreement to purchase substantially all the assets of Potentia Pharmaceuticals, Inc, for an exclusive, worldwide license to specified patent rights. The Company is required to pay annual maintenance fees of $100,000 until the first sale of a licensed product. The Company is also required to make milestone payments aggregating up to $3,200,000 based upon the achievement of specified development and regulatory milestones and up to $5,000,000 based upon the achievement of specified annual sales milestones with respect to each licensed product, and to pay low single-digit royalties based on net sales of each licensed product and with minimum quarterly royalty thresholds. In addition, the Company is obligated to pay a specified portion of income it receives from sublicensees. In addition, the Company is also party to a 2010 license agreement with Penn for an exclusive, worldwide license to specified patent rights for the development and commercialization of products in fields of use, as defined therein. The Company is required to pay annual maintenance fees of $100,000 until the first sale of a licensed product. The Company is required to make milestone payments aggregating up to $1,650,000, based upon the achievement of development and regulatory approval milestones, and up to $2,500,000, based upon the achievement of annual sales milestones with respect to each of the first two licensed products. There have been no events to date that would trigger milestone obligations under the agreement. The license agreement also requires the Company to pay low single digit royalties based on net sales of each licensed product, subject to minimum quarterly royalty thresholds. In addition, the Company is obligated to pay a specified portion of income it receives from sublicensees.

401(k) Profit Sharing Plan and Trust	12 Months Ended
401(k) Profit Sharing Plan and Trust	Dec. 31, 2017
Compensation And Retirement Disclosure [Abstract]
401(k) Profit Sharing Plan and Trust	9. 401(k) Profit Sharing Plan and Trust In July 2010, the Company adopted an employee profit-sharing plan (the “401(k) Plan”), qualified under Section 401(k) of the Internal Revenue Code (the “IRC”). All of the Company’s full-time employees who have attained the age of 21 are eligible to participate in the 401(k) Plan immediately upon employment. Pursuant to the 401(k) Plan, employees may elect to reduce their current compensation by up to the statutorily prescribed annual limit and have the amount of the reduction contributed to the 401(k) Plan. In 2016 and 2017, the Company recorded $50,867 and $123,264, respectively, for employer contributions made to the 401(k) Plan.

Refundable Research Development and Income Taxes

12 Months Ended

Dec. 31, 2017

Text Block [Abstract]

Refundable Research Development and Income Taxes

10. Refundable Research Development 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 years ended December 31, 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.

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

	Year Ended December
	2017						2016						2015
	Amount			Percentage of income before income taxes			Amount			Percentage of income before income taxes			Amount			Percentage of income before income taxes
Statutory U.S. federal income tax	$	(17,852,133	)		35.0	%	$	(9,222,377	)		34.0	%	$	(15,815,425	)		34.0	%
State income taxes, net of federal benefit		(1,634,924	)		3.3			(992,768	)		3.5			(685,158	)		1.5
Change in valuation allowances		11,176,291			(20.8	)		12,923,538			(46.4	)		6,932,927			(14.9	)
Re-measurement of deferred taxes		15,659,916			(31.9	)		—			—			—			—
Tax credits		(3,316,729	)		7.7			(2,933,836	)		10.8			(488,588	)		1.1
Acquisition Accounting		—			—			—			—			9,005,240			(19.4	)
Permanent and other		(4,032,421	)		6.7			225,443			(1.9	)		1,051,004			(2.3	)
Effective income tax provision	$	—			—	%	$	—			—	%	$	—			—	%

Deferred income taxes reflect the net tax effects of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes. Significant components of the Company’s deferred tax assets and liabilities are as follows:

	December 31,
	2016			2017
Deferred tax assets:
Accrual to cash adjustment	$	914,627		$	345,623
Intangible assets		41,166			23,543
Share-based compensation		1,000,887			1,826,372
Contribution carryforwards		23,371			24,465
Net operating loss carryforwards		22,704,142			30,699,411
Research and development credits		1,842,200			3,357,291
Orphan drug credits		3,502,686			4,928,666
Total deferred tax assets		30,029,079			41,205,371
Less valuation allowance		(30,029,079	)		(41,205,371	)
Net deferred tax assets	$	—		$	—

At December 31, 2016 and December 31, 2017, the Company had approximately $60,214,000 and $118,843,000, respectively, of net operating loss carryforwards. The Company also had $8,285,957 of federal research and development tax credit carryforwards as of December 31, 2017. The net operating loss and research and development tax credit carryforwards begin to expire in 2024.

The net operating loss and tax credit carryforwards are subject to review and possible adjustment by the Internal Revenue Service and state tax authorities. Net operating loss and tax credit carryforwards may become subject to an annual limitation in the event of certain cumulative changes in the ownership interest of significant stockholders over a three-year period in excess of 50%, as defined under Sections 382 and 383 of the IRC, respectively, as well as similar state provisions.

Under Section 382 of the Internal Revenue Code, as amended (“Section 382”), the Company’s net operating loss carryforwards (“NOLs”) and other deferred tax assets can generally be used to offset future taxable income and therefore reduce federal income tax obligations. However, the Company's ability to use its NOLs would be limited if there was an “ownership change” as defined by Section 382. This would occur if shareholders owning (or deemed to own under the tax rules) 5% or more of the Company's common stock increase their aggregate ownership of the common stock of the Company by more than 50 percentage points over a defined period of time. The Company had an ownership change on September 8, 2015. As a result, a portion of the NOL and tax credit carryforwards are subject to an annual utilization limitation.

The deferred tax asset includes approximately $30.7 million of NOLs and $8.3 million of research & development tax credit carryforwards. Of the $30.7 million of NOLs, approximately $12.3 million is limited under Section 382. Of the $8.3 million tax credit carryforwards, approximately $1.9 million is limited under Section 383. Subsequent ownership changes may further affect the limitation in future years.

Our estimate of the realizability of the deferred tax asset is dependent on our estimate of projected future levels of taxable income. In analyzing future taxable income levels, we considered all evidence currently available, both positive and negative. Based on our analysis, we have recorded a valuation allowance for all deferred tax assets as of December 31, 2016 and 2017.

Tax Act Enactment

On December 22, 2017, the U.S. government enacted comprehensive federal tax legislation commonly referred to as the Tax Cuts and Jobs Act of 2017 (the "Tax Act"). The Tax Act significantly modifies the U.S. corporate income tax system by, among other things, reducing the federal income tax rate from 35% to 21% beginning in 2018, imposing a mandatory one-time deemed repatriation tax on accumulated foreign earnings and creating a territorial tax system that changes the manner in which future foreign earnings are subject to U.S. tax. The Tax Act also provides that undistributed and previously untaxed post-1986 foreign earnings will be deemed distributed in 2017 and be subject to tax at reduced effective rates (the “Transition Tax”). The Company has a net cumulative deficit in earnings and profits from its foreign subsidiaries and, consequently, will not be subject to the Transition Tax. The company re-measured certain net deferred tax assets and liabilities based on the tax rates at which they are expected to reverse in the future. The total impact to the gross deferred tax asset balance (before valuation allowance) upon enactment of the Tax Act is $16 million, 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 estimated impacts of the Tax Act recorded during the year ended December 31, 2017 are provisional in nature, and the Company will continue to assess the impact of the U.S. Tax Reform Act and will record adjustments through the income tax provision in the relevant period as authoritative guidance is made available to the public.

The Company has generated research credits and orphan drug credits but has not conducted a detailed study to document its qualified activities. A detailed study could result in an adjustment to the Company’s research and development credit carryforwards; however, until such a study is completed and any adjustment is identified, no amounts are being presented as an uncertain tax position.

The Company does not have any unrecognized tax benefits during any periods presented and does not expect this to significantly change in the next twelve months.

There were no interest and penalties recorded in the statement of operations during any period and no amounts accrued for interest and penalties at December 31, 2017, or 2016.

The Company files income tax returns in the U.S. federal jurisdiction, and applicable state jurisdictions. The Company’s 2014 through 2017 tax years remain open and subject to examination by federal and state taxing authorities. Federal and state net operating losses are subject to review by taxing authorities in the year utilized.

Commitments and Contingencies

12 Months Ended

Dec. 31, 2017

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

11. Commitments and Contingencies

The Company leases office space in Crestwood, Kentucky under the terms of an operating lease, as amended. The lease expires in July 2018. In October 2016, the Company leased office space in Cambridge, Massachusetts under the terms of an operating lease. The lease expires in October 2018. 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. Lease expense for the years ended December 31, 2015, 2016, and 2017 was $81,521, $117,222 and $351,468, respectively.

At December 31, 2017, the Company’s future minimum payments required under these leases are as follows:

2018	$	390,344
2019		260,196
2020		226,152
2021		232,278
2022		137,580
	$	1,246,550

The Company contracts to conduct research and development activities with remaining contract costs of approximately $1,713,000 at December 31, 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.

Indemnifications—In the ordinary course of business, the Company enters into agreements that may include indemnification provisions. Pursuant to such agreements, the Company may indemnify, hold harmless and defend indemnified parties for losses suffered or incurred by the indemnified party. Some of the provisions will limit losses to those arising from third-party actions. In some cases, the indemnification will continue after the termination of the agreement. The maximum potential amount of future payments the Company could be required to make under these provisions is not determinable. The Company has never incurred any cost to defend lawsuits or settle claims related to these indemnification provisions.

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

Equity Incentive Plans

12 Months Ended

Dec. 31, 2017

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Equity Incentive Plans

12. Equity Incentive Plans

Share-based Compensation

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

In October 2017, the Company’s Board of Directors adopted, and its stockholders approved, the 2017 Stock Incentive Plan (the “2017 Plan”), which became effective on November 8, 2017. The 2017 Plan provides for the grant of incentive stock options, non-statutory stock options, stock appreciation rights, awards of restricted stock, restricted stock units and other stock-based awards. The number of shares of common stock reserved for issuance under the 2017 plan is the sum of (i) 1,359,587 shares of common stock, plus (ii) an additional number of shares of common stock equal to the sum of (a) the number of shares of common stock reserved for issuance under the 2010 equity incentive plan that remained available for future issuance immediately prior to the effectiveness of the 2017 Plan, which was 299,568 shares, and (b) the number of shares of common stock subject to outstanding awards under the 2010 equity incentive plan upon effectiveness of the 2017 plan that expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased by us at their original issuance price pursuant to a contractual repurchase right plus (iii) an annual increase, to be added the first day of each fiscal year, beginning with the fiscal year ending December 31, 2018 and continuing until, and including, the fiscal year ending December 31, 2027, equal to the lowest of 4,219,409 shares of common stock, 4.0% of the number of shares of common stock outstanding on the first day of the fiscal year and an amount determined by the board of directors.

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

The Company’s options granted prior to February 2016 become fully vested upon the occurrence of a change in control, as defined in the 2010 Plan. Options granted to executives after February 2016 become vested and exercisable with respect to 50% of the then unvested options if such executive is terminated without cause or resigns for good reason within 12 months following a change of control. Options granted to Dr. Francois, Dr. Deschatelets and Mr. Sullivan will become fully vested upon the occurrence of a change in control. The balance of the Company’s options do not receive additional vesting upon a change of control.

Effective January 22, 2016, the Board of Directors approved a modification in the exercise price of stock options to purchase 93,762 shares of common stock that were granted under the 2010 Plan to reduce the exercise price per share to $3.76 per share, which was the estimated fair market value of the common stock on the effective date of the repricing. Other stock options granted under the 2010 Plan were excluded from this repricing and will maintain their original exercise prices. The stock options that were repriced had been granted with an exercise price greater than the estimated fair market value in July and September 2015 (i.e., exercise prices ranging from $6.81 to $6.89 per share). Because the exercise prices of these stock options exceeded the estimated fair market value of the Company’s common stock on the modification date, the Board of Directors determined that the retentive value of these awards had substantially diminished from the time they had been granted. The Board of Directors determined that this repricing was in the best interests of the Company and its stockholders to provide a continued incentive for highly qualified employees and consultants with substantial experience in the Company’s business to remain employed during a critical period for the Company.

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

				Weighted -		Weighted -		Weighted -
				Average		Average		Average
				Exercise		Grant Date		Contractual		Aggregate
				Price		Fair Value		Life		Intrinsic
	Shares			Per Share		Per Share		(in years)		Value
Outstanding, January 1, 2015		2,343,090		$	2.50	$	1.56		7.52	$	578,128
Granted		210,970			4.82		3.26		9.50		65,000
Exercised		(732	)		2.35		1.43		—		1,031
Forfeited		(79,113	)		2.13		1.54		—		128,250
Outstanding, December 31, 2015		2,474,215			2.71		1.71		7.01		2,870,775
Granted		1,380,463			3.26		1.98		9.37		—
Forfeited		(23,441	)		2.43		1.13		9.83		—
Outstanding, December 31, 2016		3,831,237			2.84		1.75		7.20		243,000
Granted		2,940,444			6.94		3.61		9.67		41,878,351
Exercised		(46,881	)		3.51		2.25		—		852,818
Forfeited		(369,219	)		2.56		1.22		—		7,066,915
Outstanding, December 31, 2017		6,355,581			4.78		2.66		7.64		107,534,863
Options exercisable, December 31, 2015		1,519,866			2.43		1.58		6.10		2,058,419
Expected to vest, December 31, 2015		954,349			3.16		1.90		8.46		812,365
Options exercisable, December 31, 2016		2,048,318			2.56		1.64		5.76		243,000
Expected to vest, December 31, 2016		1,782,919			3.16		1.90		8.85		—
Options exercisable, December 31, 2017		3,156,433			3.67		2.12		5.90		56,918,296
Expected to vest, December 31, 2017		3,199,149			5.88		3.19		9.35		50,616,567

The aggregate intrinsic values of options outstanding, exercisable, vested and expected to vest were calculated as the difference between the exercise price of the options and the estimated fair value of the common stock as of December 31, 2017. Estimated fair values of the common stock at the time of the grants between May 12, 2010 and December 31, 2017 were between $1.71 and $14.00.

Total share-based compensation expense recognized was as follows:

	Year Ended December 31,
	2015		2016		2017
Research and development	$	171,388	$	377,776	$	2,678,956
General and administrative		374,313		701,212		2,739,754
Total share-based compensation expense	$	545,701	$	1,078,988	$	5,418,710

At December 31, 2015, 2016 and 2017, unrecognized compensation expense related to unvested options, net of estimated forfeitures, was $1,673,110, $2,910,870 and $10,415,787, respectively, which the Company expects to recognize over an estimated weighted-average period of 2.36, 2.85, and 3.14 years, respectively. As of December 31, 2017, the future amortization of unearned share-based compensation costs will be $4,693,212 in 2018, $2,548,514 in 2019, $1,949,248 in 2020 and $1,224,813 in 2021.

The assumptions used in the Monte Carlo and Black-Scholes models to estimate the grant date fair value are as follows:

	Year Ended December 31,
	2015	2016	2017
Risk-free interest rate	1.61 - 1.87%	1.21 - 1.60%	1.21 - 2.30%
Dividend yield	0%	0%	0%
Volatility	78.1 - 93.5%	52.0 - 78.1%	51.6 - 55.8%
Expected terms (years)	5.4 - 6.2	5.2 - 5.7	5.3 - 7.0

2017 Employee Stock Purchase Plan

In October 2017, the Company’s board of directors adopted and the Company’s stockholders approved the 2017 Employee Stock Purchase Plan (“ESPP”), which became effective upon the IPO and provides participating employees with the opportunity to purchase up to an aggregate of 468,823 shares of our common stock. The number of shares of our common stock reserved for issuance under the 2017 ESPP will automatically increase on the first day of each fiscal year, beginning with the fiscal year ending December 31, 2018 and continuing until, and including, the fiscal year ending December 31, 2027, equal to the lowest of (i) 937,646 shares of our common stock, (ii) 1.0% of the number of shares of our common stock outstanding on the first day of the fiscal year and (iii) an amount determined by our board of directors. The board of directors has not initiated any offerings under the ESPP.

Net Loss per Common Share

12 Months Ended

Dec. 31, 2017

Earnings Per Share [Abstract]

Net Loss per Common Share

13. Net Loss per Common Share

The following table presents the calculation of basic and diluted net loss per common share:

	Year Ended December 31,
	2015			2016			2017
Numerator:
Net loss and comprehensive loss	$	(46,515,956	)	$	(27,124,637	)	$	(51,006,094	)
Denominator:
Weighted-average number of common shares used in net loss per common share -- basic and diluted		5,795,040			8,428,366			13,870,949
Net loss per common share -- basic and diluted	$	(8.03	)	$	(3.22	)	$	(3.68	)

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:

	Year Ended December 31,
	2015		2016		2017
Convertible preferred stock		23,269,107		26,416,896		—
Common stock under option		2,474,215		3,831,237		6,355,581
Common stock warrants		—		—		14,064
Total		25,743,322		30,248,133		6,369,645

Selected Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Selected Quarterly Financial Data (Unaudited)

14. Selected Quarterly Financial Data (Unaudited)

The following interim financial information presents the Company’s 2016 and 2017 results of operations on a quarterly basis:

	Quarter Ended
	March 31,			June 30,			September 30,			December 31,
	2016			2016			2016			2016
Net Operating Loss	$	(6,228,452	)	$	(7,572,638	)	$	(7,234,204	)	$	(6,247,048	)
Net Loss		(5,999,699	)		(7,294,245	)		(7,210,591	)		(6,620,102	)
Net Loss per common share, basic
and diluted		(0.33	)		(0.41	)		(0.86	)		(3.22	)

	Quarter Ended
	March 31,			June 30,			September 30,			December 31,
	2017			2017			2017			2017
Net Operating Loss	$	(9,214,375	)	$	(12,394,490	)	$	(11,589,319	)	$	(17,568,845	)
Net Loss		(9,050,773	)		(12,126,152	)		(11,573,273	)		(18,255,896	)
Net Loss per common share, basic
and diluted		(0.50	)		(0.67	)		(1.37	)		(3.68	)

Subsequent Events	12 Months Ended
Subsequent Events	Dec. 31, 2017
Subsequent Events [Abstract]
Subsequent Events	15. Subsequent Events In February 2018, the Company granted an aggregate of 1,126,585 options to purchase shares of common stock to employees with an exercise price ranging from $14.64 to $16.45.

Summary of Significant Accounting Policies (Policies)	12 Months Ended
Summary of Significant Accounting Policies (Policies)	Dec. 31, 2017
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying 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 consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and following the requirements of the Securities and Exchange Commission (the “SEC”).
Reclassifications	Reclassifications Certain prior year amounts have been reclassified to conform to the 2017 presentation.
Stock Split	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 the Company’s then outstanding series of convertible preferred stock to reflect a proportional decrease in the number of shares of common stock to be issued upon conversion. The accompanying 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” an amount equal to the par value of the decreased shares resulting from the reverse stock split.
Offering Costs	Offering Costs Offering costs represent underwriting, legal, accounting and other direct costs related to the Company’s IPO. These costs were deferred until completion of the IPO, at which time they were reclassified to additional paid-in capital as a reduction of the proceeds.
Segment Information	Segment Information Operating segments are defined as components of an enterprise about which separate discrete information is available for evaluation by the chief operating decision maker, or decision-making group, in deciding how to allocate resources and in assessing performance. The Company views its operations and manages its business in one operating segment.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results may differ from those estimates. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates, and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes, and management must select an amount that falls within that range of reasonable estimates. Estimates are used in the following areas, among others: share-based compensation expense, fair value of common stock and preferred stock, accrued expenses, prepaid expenses and income taxes. Historically for all periods prior to the IPO, the Company utilized various valuation methodologies in accordance with the framework of the American Institute of Certified Public Accountants Technical Practice Aid, Valuation of Privately-Held Company Equity Securities Issued as Compensation, to estimate the retrospective fair value of its common stock during all periods presented. The methodologies include a probability analysis including both a potential public trading scenario and potential sale scenario. In both scenarios, value is estimated using the guideline public company method. The sale scenario includes an adjustment for a market participant acquisition premium. Value is allocated among the preferred and common shares according to the rights associated with each type of security. Valuation methodologies include estimates and assumptions that require the Company’s judgment. These estimates include assumptions regarding future performance. Significant changes to the key assumptions used in the valuations could result in different fair values of common stock and the associated fair value of stock options granted at each valuation date. Since the completion of the IPO, the Company’s board of directors determines the fair value of underlying common stock based upon the closing price of the Company’s common stock as reported on the date of grant.
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.
Cash and Cash Equivalents	Cash and Cash Equivalents Cash and cash equivalents are defined as cash in banks and investment instruments having maturities of three months or less from their acquisition date. The carrying amounts reported in the consolidated balance sheets for cash and cash equivalents are valued at cost, which approximates their fair value.
Foreign Currency	Foreign Currency The functional currency of the Company’s wholly-owned subsidiary is the U.S. dollar.
Research and Development	Research and Development Costs incurred in connection with research and development activities are expensed as incurred. Research and development expenses include (i) employee-related expenses, including salaries, benefits, travel and share-based compensation expense; (ii) external research and development expenses incurred under arrangements with third parties, such as contract research and contract manufacturing organizations, investigational sites and consultants, including share-based compensation expense for consultants; (iii) the cost of acquiring, developing and manufacturing clinical study materials; and (iv) costs associated with preclinical and clinical activities and regulatory operations. The Company enters into consulting, research and other agreements with commercial entities, researchers, universities and others for the provision of goods and services. Such arrangements are generally cancellable upon reasonable notice and payment of costs incurred. Costs are considered incurred based on an evaluation of the progress to completion of specific tasks under each contract using information and data provided by the Company’s clinical sites and vendors. These costs consist of direct and indirect costs associated with specific projects, as well as fees paid to various entities that perform certain research on behalf of the Company. Depending upon the timing of payments to the service providers, the Company recognizes prepaid expenses or accrued expenses related to these costs. These accrued or prepaid expenses are based on management’s estimates of the work performed under service agreements, milestones achieved and experience with similar contracts. The Company monitors each of these factors and adjusts estimates accordingly. Included in research and development is a refundable Australian research and development credit. The credit is recognized as a reduction of clinical trial costs over the periods necessary to match the benefit of the credit with the costs for which it is intended to compensate. During 2017, the Company identified and corrected an immaterial presentation error in the consolidated statements of operations and comprehensive loss for the years ended December 31, 2015 and 2016. The error was the result of presenting $1.7 and $1.2 million of Australian research and development refundable credits for the years ended December 31, 2015 and 2016, respectively as an income tax benefit in the period. 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, other than the Company’s tax footnote.
Patents	Patents Costs incurred in connection with the application for and issuance of patents are expensed as incurred.
Income Taxes	Income Taxes Income taxes are recorded in accordance with Financial Accounting Standards Board (“FASB”) ASC Topic 740, Income Taxes (“ASC 740”), which provides for deferred taxes using an asset and liability approach. The Company recognizes deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements or tax returns. Deferred tax assets and liabilities are determined based on the difference between the financial statement and tax bases of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Valuation allowances are provided if, based upon the weight of available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized. The Company accounts for uncertain tax positions in accordance with the provisions of ASC 740. When uncertain tax positions exist, the Company recognizes the tax benefit of tax positions to the extent that the benefit will more likely than not be realized. The determination as to whether the tax benefit will more likely than not be realized is based upon the technical merits of the tax position, as well as consideration of the available facts and circumstances. As of December 31, 2016 and 2017, the Company did not have any significant uncertain tax positions.
Share-based Compensation	Share-based Compensation The Company accounts for its share-based compensation awards in accordance with FASB ASC Topic 718, Compensation—Stock Compensation (“ASC 718”). ASC 718 requires all share-based payments to employees, including grants of employee stock options, be recognized in the statements of operations and comprehensive loss based on their estimated fair values over the requisite service periods for each award. The Company accounts for share-based awards to non-employees in accordance with FASB ASC Topic 505-50, Equity-Based Payments to Non-Employees (“ASC 505-50”), which requires the fair value of the award to be re-measured at fair value until a performance commitment is reached or counterparty performance is complete. The Company’s share-based compensation awards are comprised of stock options. 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 through October 2017. After the Company’s IPO in November 2017, the Company’s board of directors determined the fair value of underlying common stock based upon the closing price of the Company’s common stock as reported on the date of the grant. Both Monte Carlo and Black-Scholes for option pricing require the input of the six minimum considerations detailed in ASC 718, including (a) the expected stock price volatility, (b) the calculation of expected term of the award, (c) the risk-free interest rate and (d) expected dividends. Due to the lack of a public market for the trading of the Company’s common stock and a lack of company-specific historical and implied volatility data, the Company has based its estimate of expected volatility on the historical volatility of a group of comparable companies that are publicly traded. The historical volatility is calculated based on a period of time commensurate with the expected term assumption. The computation of expected volatility is based on the historical volatility of a representative group of companies with similar characteristics to the Company, including stage of product development and life science industry focus. The Company is in a very early stage of product development with no revenues, and the representative group of companies has similar characteristics. The Company believes the group selected has sufficient similar economic and industry characteristics. The Company calculated the expected term for options granted to employees based on a quarterly weighted average probability of exit analysis considering milestones that the Company had achieved and each of the potential exit scenarios available to the Company at that time. The expected term is applied to the stock option grant group as a whole, as the Company does not expect substantially different exercise or post-vesting termination behavior among its employee population. For options granted to non-employees, the Company utilizes the contractual term of the arrangement as the basis for the expected term assumption. The risk-free interest rate is based on a treasury instrument whose term is consistent with the expected life of the stock options. The expected dividend yield is assumed to be zero as the Company has never paid dividends and has no current plans to pay any dividends on its common stock. The Company’s share-based awards are subject to service based vesting conditions. Compensation expense related to awards to employees with service based vesting conditions is recognized on a straight-line basis based on the grant date fair value over the associated service period of the award, which is generally the vesting term. Consistent with the guidance in ASC 505-50, compensation expense related to awards to non-employees with service based vesting conditions is recognized on a straight-line basis based on the then-current fair value at each financial reporting date prior to the measurement date over the associated service period of the award, which is generally the vesting term. The Company recognizes forfeitures for employee and non-employee grants at the time they occur. Share-based compensation expense recognized in the financial statements is based on awards that are ultimately expected to vest.
Concentrations of Credit Risk	Concentrations of Credit Risk Cash and accounts receivable are the only financial instruments that potentially subject the Company to concentrations of credit risk. The Company’s cash is maintained with accredited financial institutions and is insured by the Federal Deposit Insurance Corporation.
Net Loss per Share	Net Loss per Share Basic net loss per common share is calculated by dividing net loss by the weighted-average shares outstanding during the period. For purposes of the diluted net loss per share calculation, convertible preferred stock and common stock options are considered to be common stock equivalents but have been excluded from the calculation of diluted net loss per share, as their effect would be anti-dilutive for all periods presented. Therefore, basic and diluted net loss per share were the same for all periods presented.
Comprehensive Loss	Comprehensive Loss The Company has no components of comprehensive loss other than its net loss and, accordingly, comprehensive loss is the same as the net loss for all periods presented.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In May 2014, the 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. 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: Balance Sheet Classification of Deferred Taxes. This ASU simplifies the presentation of deferred income taxes and requires that deferred tax liabilities and assets be classified 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 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 for 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.

Common Stock (Tables)

12 Months Ended

Dec. 31, 2017

Equity [Abstract]

Summary of Shares of Common Stock Reserved for Future Issuance

The Company has reserved the following shares of common stock for future issuance:

	December 31,
	2015		2016		2017
Conversion of Series A convertible preferred stock		1,251,758		1,251,758		—
Conversion of Series B convertible preferred stock		2,982,962		2,982,962		—
Conversion of Series C convertible preferred stock		12,290,394		12,290,394		—
Conversion of Series D convertible preferred stock		6,743,993		9,891,866		—
Shares reserved under 2010 Equity Incentive Plan		3,375,527		4,781,264		5,817,844
Shares reserved under 2017 Equity Incentive Plan		—		—		1,682,596
Shares reserved under 2017 Employee Stock Purchase Plan						468,823
Common stock warrants		—		—		14,064
Total		26,644,634		31,198,244		7,983,327

Accrued Expenses (Tables)

12 Months Ended

Dec. 31, 2017

Text Block [Abstract]

Schedule of Accrued Expenses

Accrued expenses are as follows:

	December 31,
	2016		2017
Accrued research and development	$	871,066	$	2,463,808
Accrued payroll liabilities		91,598		251,491
Other		129,062		175,406
Total	$	1,091,726	$	2,890,705

Long-term Debt (Tables)

12 Months Ended

Dec. 31, 2017

Debt Disclosure [Abstract]

Summary of Contractual Maturities of Long-term Debt Obligations

The contractual maturities of our long-term debt obligations due subsequent to December 31, 2017 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 Development and Income Taxes (Tables)

12 Months Ended

Dec. 31, 2017

Refundable Research Development And Income Taxes [Abstract]

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

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

	Year Ended December
	2017						2016						2015
	Amount			Percentage of income before income taxes			Amount			Percentage of income before income taxes			Amount			Percentage of income before income taxes
Statutory U.S. federal income tax	$	(17,852,133	)		35.0	%	$	(9,222,377	)		34.0	%	$	(15,815,425	)		34.0	%
State income taxes, net of federal benefit		(1,634,924	)		3.3			(992,768	)		3.5			(685,158	)		1.5
Change in valuation allowances		11,176,291			(20.8	)		12,923,538			(46.4	)		6,932,927			(14.9	)
Re-measurement of deferred taxes		15,659,916			(31.9	)		—			—			—			—
Tax credits		(3,316,729	)		7.7			(2,933,836	)		10.8			(488,588	)		1.1
Acquisition Accounting		—			—			—			—			9,005,240			(19.4	)
Permanent and other		(4,032,421	)		6.7			225,443			(1.9	)		1,051,004			(2.3	)
Effective income tax provision	$	—			—	%	$	—			—	%	$	—			—	%

Schedule of Deferred Tax Assets and Liabilities

Significant components of the Company’s deferred tax assets and liabilities are as follows:

	December 31,
	2016			2017
Deferred tax assets:
Accrual to cash adjustment	$	914,627		$	345,623
Intangible assets		41,166			23,543
Share-based compensation		1,000,887			1,826,372
Contribution carryforwards		23,371			24,465
Net operating loss carryforwards		22,704,142			30,699,411
Research and development credits		1,842,200			3,357,291
Orphan drug credits		3,502,686			4,928,666
Total deferred tax assets		30,029,079			41,205,371
Less valuation allowance		(30,029,079	)		(41,205,371	)
Net deferred tax assets	$	—		$	—

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2017

Commitments And Contingencies Disclosure [Abstract]

Schedule of Future Minimum Payments Required Under the Leases

At December 31, 2017, the Company’s future minimum payments required under these leases are as follows:

2018	$	390,344
2019		260,196
2020		226,152
2021		232,278
2022		137,580
	$	1,246,550

Equity Incentive Plans (Tables)

12 Months Ended

Dec. 31, 2017

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Stock Option Activity

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

				Weighted -		Weighted -		Weighted -
				Average		Average		Average
				Exercise		Grant Date		Contractual		Aggregate
				Price		Fair Value		Life		Intrinsic
	Shares			Per Share		Per Share		(in years)		Value
Outstanding, January 1, 2015		2,343,090		$	2.50	$	1.56		7.52	$	578,128
Granted		210,970			4.82		3.26		9.50		65,000
Exercised		(732	)		2.35		1.43		—		1,031
Forfeited		(79,113	)		2.13		1.54		—		128,250
Outstanding, December 31, 2015		2,474,215			2.71		1.71		7.01		2,870,775
Granted		1,380,463			3.26		1.98		9.37		—
Forfeited		(23,441	)		2.43		1.13		9.83		—
Outstanding, December 31, 2016		3,831,237			2.84		1.75		7.20		243,000
Granted		2,940,444			6.94		3.61		9.67		41,878,351
Exercised		(46,881	)		3.51		2.25		—		852,818
Forfeited		(369,219	)		2.56		1.22		—		7,066,915
Outstanding, December 31, 2017		6,355,581			4.78		2.66		7.64		107,534,863
Options exercisable, December 31, 2015		1,519,866			2.43		1.58		6.10		2,058,419
Expected to vest, December 31, 2015		954,349			3.16		1.90		8.46		812,365
Options exercisable, December 31, 2016		2,048,318			2.56		1.64		5.76		243,000
Expected to vest, December 31, 2016		1,782,919			3.16		1.90		8.85		—
Options exercisable, December 31, 2017		3,156,433			3.67		2.12		5.90		56,918,296
Expected to vest, December 31, 2017		3,199,149			5.88		3.19		9.35		50,616,567

Summary of Share-based Compensation Expense Recognized

Total share-based compensation expense recognized was as follows:

	Year Ended December 31,
	2015		2016		2017
Research and development	$	171,388	$	377,776	$	2,678,956
General and administrative		374,313		701,212		2,739,754
Total share-based compensation expense	$	545,701	$	1,078,988	$	5,418,710

Summary of Assumption Used to Estimate the Grant Date Fair Value

The assumptions used in the Monte Carlo and Black-Scholes models to estimate the grant date fair value are as follows:

	Year Ended December 31,
	2015	2016	2017
Risk-free interest rate	1.61 - 1.87%	1.21 - 1.60%	1.21 - 2.30%
Dividend yield	0%	0%	0%
Volatility	78.1 - 93.5%	52.0 - 78.1%	51.6 - 55.8%
Expected terms (years)	5.4 - 6.2	5.2 - 5.7	5.3 - 7.0

Net Loss per Common Share (Tables)

12 Months Ended

Dec. 31, 2017

Earnings Per Share [Abstract]

Summary of Basic and Diluted Net Loss per Common Share

The following table presents the calculation of basic and diluted net loss per common share:

	Year Ended December 31,
	2015			2016			2017
Numerator:
Net loss and comprehensive loss	$	(46,515,956	)	$	(27,124,637	)	$	(51,006,094	)
Denominator:
Weighted-average number of common shares used in net loss per common share -- basic and diluted		5,795,040			8,428,366			13,870,949
Net loss per common share -- basic and diluted	$	(8.03	)	$	(3.22	)	$	(3.68	)

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:

	Year Ended December 31,
	2015		2016		2017
Convertible preferred stock		23,269,107		26,416,896		—
Common stock under option		2,474,215		3,831,237		6,355,581
Common stock warrants		—		—		14,064
Total		25,743,322		30,248,133		6,369,645

Selected Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Summary of Interim Financial Information on Quarterly Basis

The following interim financial information presents the Company’s 2016 and 2017 results of operations on a quarterly basis:

	Quarter Ended
	March 31,			June 30,			September 30,			December 31,
	2016			2016			2016			2016
Net Operating Loss	$	(6,228,452	)	$	(7,572,638	)	$	(7,234,204	)	$	(6,247,048	)
Net Loss		(5,999,699	)		(7,294,245	)		(7,210,591	)		(6,620,102	)
Net Loss per common share, basic
and diluted		(0.33	)		(0.41	)		(0.86	)		(3.22	)

	Quarter Ended
	March 31,			June 30,			September 30,			December 31,
	2017			2017			2017			2017
Net Operating Loss	$	(9,214,375	)	$	(12,394,490	)	$	(11,589,319	)	$	(17,568,845	)
Net Loss		(9,050,773	)		(12,126,152	)		(11,573,273	)		(18,255,896	)
Net Loss per common share, basic
and diluted		(0.50	)		(0.67	)		(1.37	)		(3.68	)

Nature of Organization and Operations - Additional Information (Detail) - IPO [Member] - USD ($) $ / shares in Units, $ in Millions	Dec. 13, 2017	Nov. 13, 2017
Nature of Organization and Operations [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	$ 12.8	$ 137.2
Underwriting discounts and commissions	$ 1.0	10.5
Initial public offering costs		$ 2.3

Summary of Significant Accounting Policies - Additional Information (Detail)		12 Months Ended
	Oct. 27, 2017 $ / shares	Dec. 31, 2017 USD ($) Segment $ / shares	Dec. 31, 2016 USD ($) $ / shares	Dec. 31, 2015 USD ($)
Schedule Of 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
Common stock, par value \| $ / shares	$ 0.0001	$ 0.0001	$ 0.0001
Reverse stock split, effective date	Oct. 27, 2017
Number of operating segment \| Segment		1
Refundable research and development credit		$ 50,443	$ 408,934	$ (1,313,398)
Revenues on initial stage of product development		0
Expected dividend payments		$ 0
Expected dividend yield		0.00%	0.00%	0.00%
Error Adjustment
Schedule Of Summary Of Significant Accounting Policies [Line Items]
Refundable research and development credit			$ 1,200,000	$ 1,700,000

Asset Purchase From Related Party - Additional Information (Detail) - USD ($)	1 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2015
Related Party Transaction [Line Items]
In-process research and development expense			$ 26,486,000
Potentia Pharmaceuticals, Inc. [Member] \| Vendor Payments under Asset Purchase Agreement [Member]
Related Party Transaction [Line Items]
Expenses recognized related to asset purchase agreement			$ 874,427
Potentia Pharmaceuticals, Inc. [Member] \| Common Stock [Member]
Related Party Transaction [Line Items]
Assets purchased in exchange of common stock, shares		3,844,352
Acquisition of assets and shares issued, fair value	$ 26,486,000

Common Stock - Summary of Shares of Common Stock Reserved for Future Issuance (Detail) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class Of Stock [Line Items]
Common stock reserved for future issuance	7,983,327	31,198,244	26,644,634
Common stock warrants	14,064
2010 Stock Incentive Plan [Member]
Class Of Stock [Line Items]
Common stock reserved for future issuance	5,817,844	4,781,264	3,375,527
2017 Stock Incentive Plan [Member]
Class Of Stock [Line Items]
Common stock reserved for future issuance	1,682,596
2017 Employee Stock Purchase Plan [Member]
Class Of Stock [Line Items]
Common stock reserved for future issuance	468,823
Series A Convertible Preferred Stock [Member]
Class Of Stock [Line Items]
Common stock reserved for future issuance		1,251,758	1,251,758
Series B Convertible Preferred Stock [Member]
Class Of Stock [Line Items]
Common stock reserved for future issuance		2,982,962	2,982,962
Series C Convertible Preferred Stock [Member]
Class Of Stock [Line Items]
Common stock reserved for future issuance		12,290,394	12,290,394
Series D Convertible Preferred Stock [Member]
Class Of Stock [Line Items]
Common stock reserved for future issuance		9,891,866	6,743,993

Convertible Preferred Stock - Additional Information (Detail) - USD ($)	1 Months Ended				3 Months Ended	12 Months Ended
	Aug. 31, 2017	Jan. 31, 2016	Dec. 31, 2015	Jan. 31, 2015	May 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class Of Stock [Line Items]
Authorized capital stock						10,000,000	0
Issuance of convertible preferred stock						0	0
Convertible preferred stock issued, price per share						$ 0.0001	$ 0.0001
Convertible preferred stock issuance costs								$ 90,868
Convertible Preferred Stock [Member]
Class Of Stock [Line Items]
Authorized capital stock							56,347,420
Series C Convertible Preferred Stock [Member]
Class Of Stock [Line Items]
Authorized capital stock						0	26,215,411
Issuance of convertible preferred stock				183,333	6,000,000	0	26,215,411
Convertible preferred stock issued, price per share				$ 1.50	$ 1.50	$ 0.0001	$ 0.0001
Gross proceeds from issuance and sale of convertible preferred stock				$ 275,000
Net proceeds from issuance and sale of convertible preferred stock					$ 9,000,000			$ 9,275,001
Series D Convertible Preferred Stock [Member]
Class Of Stock [Line Items]
Authorized capital stock						0	21,099,351
Issuance of convertible preferred stock		6,714,413	14,384,938			0	21,099,351	14,384,938
Convertible preferred stock issued, price per share		$ 2.234	$ 2.234			$ 0.0001	$ 0.0001	$ 2.234
Gross proceeds from issuance and sale of convertible preferred stock			$ 32,135,951
Net proceeds from issuance and sale of convertible preferred stock		$ 15,000,000					$ 14,863,020	$ 32,050,646
Convertible preferred stock issuance costs		$ 136,980	$ 85,306
Series E Convertible Preferred Stock [Member]
Class Of Stock [Line Items]
Issuance of convertible preferred stock	7,792,035
Convertible preferred stock issued, price per share	$ 2.571
Gross proceeds from issuance and sale of convertible preferred stock	$ 20,033,322
Net proceeds from issuance and sale of convertible preferred stock						$ 19,747,847
Convertible preferred stock issuance costs	$ 285,475

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Accrued Liabilities Current [Abstract]
Accrued research and development	$ 2,463,808	$ 871,066
Accrued payroll liabilities	251,491	91,598
Other	175,406	129,062
Total	$ 2,890,705	$ 1,091,726

Long-term Debt - Additional Information (Detail)			12 Months Ended
Long-term Debt - Additional Information (Detail)	Oct. 20, 2017 USD ($) Installment $ / shares shares	Oct. 19, 2017 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) shares
Debt Instrument [Line Items]
Warrant to purchase common stock \| shares			14,064
Fair value of warrant			$ 153,692
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, thereafter from November 1, 2019 through October 1, 2021, at which time 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)	Dec. 31, 2017 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

License Agreements - Additional Information (Detail)	1 Months Ended	12 Months Ended
License Agreements - Additional Information (Detail)	Sep. 30, 2014 USD ($)	Dec. 31, 2017 USD ($) License
University of Pennsylvania [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Annual maintenance fees	$ 100,000
University of Pennsylvania [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate milestones payments	3,200,000
Milestone payments based on annual sales milestones	$ 5,000,000
2010 License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Annual maintenance fees		$ 100,000
Number of licensed products \| License		2
2010 License Agreement [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate milestones payments		$ 1,650,000
Milestone payments based on annual sales milestones		$ 2,500,000

401(k) Profit Sharing Plan and Trust - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Compensation And Retirement Disclosure [Abstract]
Minimum age of full-time employees eligilble to participate in the plan	21 years
Employer contributions made to the 401(k) Plan	$ 50,867	$ 123,264

Refundable Research Development and Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 08, 2015
Refundable Research Development And Income Taxes [Line Items]
Current income tax expense or benefit		$ 0	$ 0
Deferred income tax expense or benefit		0	0
Net operating loss carryforwards		118,843,000	$ 60,214,000
Research and development tax credit carryforwards		$ 8,285,957
Net operating loss carryforward expiration year		2024
Research and development tax credit carryforward expiration year		2024
Net operating loss carryforward period		3 years
Cumulative changes in ownership interest of significant stockholders		50.00%
Minimum increase in ownership percentage					5.00%
Deferred tax assets		$ 30,700,000
Statutory U.S. federal income tax, Percentage of income before income taxes		35.00%	34.00%	34.00%
Gross deferred tax asset balance		$ 15,659,916
Interest and penalties		0	$ 0
Accrued for interest and penalties		$ 0	$ 0
Income tax examination description		The Company’s 2014 through 2017 tax years remain open and subject to examination by federal and state taxing authorities
Scenario Plan [Member]
Refundable Research Development And Income Taxes [Line Items]
Statutory U.S. federal income tax, Percentage of income before income taxes	21.00%
IRC Sec. 382 [Member]
Refundable Research Development And Income Taxes [Line Items]
Deferred tax assets		$ 12,300,000
IRC Sec. 383 [Member]
Refundable Research Development And Income Taxes [Line Items]
Research and development tax credit carryforwards		$ 1,900,000

Refundable Reserach Development and Income Taxes - Schedule of Effective Income Tax Rate Provision Amount Differs from Statutory U.S. Federal Income Tax Rate (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Expense Benefit Continuing Operations [Abstract]
Statutory U.S. federal income tax, Amount	$ (17,852,133)	$ (9,222,377)	$ (15,815,425)
State income taxes, net of federal benefit, Amount	(1,634,924)	(992,768)	(685,158)
Change in valuation allowances, Amount	11,176,291	12,923,538	6,932,927
Re-measurement of deferred taxes, Amount	15,659,916
Tax credits, Amount	(3,316,729)	(2,933,836)	(488,588)
Acquisition Accounting, Amount			9,005,240
Permanent and other, Amount	$ (4,032,421)	$ 225,443	$ 1,051,004

Refundable Reserach Development and Income Taxes - Schedule of Effective Income Tax Rate Provision Differs Amount from Statutory U.S. Federal Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Expense Benefit Continuing Operations [Abstract]
Statutory U.S. federal income tax, Percentage of income before income taxes	35.00%	34.00%	34.00%
State income taxes, net of federal benefit, Percentage of income before income taxes	3.30%	3.50%	1.50%
Change in valuation allowances, Percentage of income before income taxes	(20.80%)	(46.40%)	(14.90%)
Re-measurement of deferred taxes, Percentage of income before income taxes	(31.90%)
Tax credits, Percentage of income before income taxes	7.70%	10.80%	1.10%
Acquisition Accounting, Percentage of income before income taxes			(19.40%)
Permanent and other, Percentage of income before income taxes	6.70%	(1.90%)	(2.30%)

Refundable Research Development and Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Detail) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Accrual to cash adjustment	$ 345,623	$ 914,627
Intangible assets	23,543	41,166
Share-based compensation	1,826,372	1,000,887
Contribution carryforwards	24,465	23,371
Net operating loss carryforwards	30,699,411	22,704,142
Research and development credits	3,357,291	1,842,200
Orphan drug credits	4,928,666	3,502,686
Total deferred tax assets	41,205,371	30,029,079
Less valuation allowance	$ (41,205,371)	$ (30,029,079)

Commitments and Contingencies - Additional Information (Detail)	1 Months Ended			12 Months Ended
	May 31, 2017 USD ($) ft²	Apr. 30, 2017 USD ($) ft²	Oct. 31, 2016	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Commitments And Contingencies [Line Items]
Base rent payable				$ 1,246,550
Remaining contract costs				$ 1,713,000
Crestwood, Kentucky [Member]
Commitments And Contingencies [Line Items]
Lease expiration date				Jul. 31, 2018
Cambridge, Massachusetts [Member]
Commitments And Contingencies [Line Items]
Lease expiration date			Oct. 31, 2018
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
Lease expense				$ 351,468	$ 117,222	$ 81,521

Commitments and Contingencies - Schedule of Future Minimum Payments Required Under the Leases (Detail)	Dec. 31, 2017 USD ($)
Commitments And Contingencies Disclosure [Abstract]
2018	$ 390,344
2019	260,196
2020	226,152
2021	232,278
2022	137,580
Total	$ 1,246,550

Equity Incentive Plans - Additional Information (Detail) - USD ($)		1 Months Ended		12 Months Ended
	Jan. 22, 2016	Oct. 31, 2017	Aug. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	May 12, 2010
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options vesting period				48 months
Stock options vesting description				Options granted to employees on or after December 5, 2013 generally vest in installments of (i) 25% at the one year anniversary and (ii) in either 36 equal monthly or 12 equal quarterly installments beginning in the thirteenth month after the initial vesting commencement date (as defined) subject to the employee’s continuous service with the Company.
Number of shares granted				2,940,444	1,380,463	210,970
Exercise price per share				$ 6.94	$ 3.26	$ 4.82
Estimated fair values of common stock, per share				$ 14.00			$ 1.71
Unrecognized compensation expense				$ 10,415,787	$ 2,910,870	$ 1,673,110
Estimated weighted-average period to recognize				3 years 1 month 20 days	2 years 10 months 6 days	2 years 4 months 9 days
Future amortization of unearned share-based compensation costs, 2018				$ 4,693,212
Future amortization of unearned share-based compensation costs, 2019				2,548,514
Future amortization of unearned share-based compensation costs, 2020				1,949,248
Future amortization of unearned share-based compensation costs, 2021				$ 1,224,813
Common stock reserved for future issuance				7,983,327	31,198,244	26,644,634
Frist Anniversary [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options vesting percentage				25.00%
Second Anniversary [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options vesting percentage				25.00%
Third Anniversary [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options vesting percentage				25.00%
Fourth Anniversary [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options vesting percentage				25.00%
Equity Incentive Plan 2010 [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock shares reserved for issuance			6,188,466
Precentage of voting shares			10.00%
Options expire from issuance date			10 years
Number of shares granted	93,762
Exercise price per share	$ 3.76
Exercise price range, lower range limit	6.81
Exercise price range, higher range limit	$ 6.89
Equity Incentive Plan 2010 [Member] \| Executive [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options vested and exercisable percentage				50.00%
Equity Incentive Plan 2010 [Member] \| Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of estimated fair value per share of common stock on the date of grant			100.00%
Equity Incentive Plan 2010 [Member] \| Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage price of each share of the fair value on date of grant			110.00%
2017 Stock Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock shares reserved for issuance		1,359,587
Precentage of voting shares		4.00%
Number of shares available for future grant		299,568
Shares issuable increased during the period		4,219,409
Common Stock Issuance Description				Equal to the lowest of 4,219,409 shares of common stock, 4.0% of the number of shares of common stock outstanding on the first day of the fiscal year and an amount determined by the board of directors.
Common stock reserved for future issuance				1,682,596
2017 Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock shares reserved for issuance		468,823
Precentage of voting shares				1.00%
2017 Employee Stock Purchase Plan [Member] \| Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance				937,646

Equity Incentive Plans - Summary of Stock Option Activity (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Shares Outstanding, Beginning balance	3,831,237	2,474,215	2,343,090
Shares, Granted	2,940,444	1,380,463	210,970
Shares, Exercised	(46,881)		(732)
Shares, Forfeited	(369,219)	(23,441)	(79,113)
Shares Outstanding, Ending balance	6,355,581	3,831,237	2,474,215	2,343,090
Shares, Options exercisable	3,156,433	2,048,318	1,519,866
Shares, Expected to vest	3,199,149	1,782,919	954,349
Weighted -Average Exercise Price Per Share Outstanding, Beginning balance	$ 2.84	$ 2.71	$ 2.50
Weighted -Average Exercise Price Per Share, Granted	6.94	3.26	4.82
Weighted -Average Exercise Price Per Share, Exercised	3.51		2.35
Weighted -Average Exercise Price Per Share, Forfeited	2.56	2.43	2.13
Weighted -Average Exercise Price Per Share, Outstanding, Ending balance	4.78	2.84	2.71	$ 2.50
Weighted -Average Exercise Price Per Share, Options exercisable	3.67	2.56	2.43
Weighted -Average Exercise Price Per Share, Expected to vest	5.88	3.16	3.16
Weighted - Average Grant Date Fair Value Per Share Outstanding, Beginning balance	1.75	1.71	1.56
Weighted - Average Grant Date Fair Value Per Share, Granted	3.61	1.98	3.26
Weighted - Average Grant Date Fair Value Per Share, Exercised	2.25		1.43
Weighted - Average Grant Date Fair Value Per Share, Forfeited	1.22	1.13	1.54
Weighted - Average Grant Date Fair Value Per Share Outstanding, Ending balance	2.66	1.75	1.71	$ 1.56
Weighted - Average Grant Date Fair Value Per Share, Options exercisable	2.12	1.64	1.58
Weighted - Average Grant Date Fair Value Per Share, Expected to vest	$ 3.19	$ 1.90	$ 1.90
Weighted - Average Contractual Life Outstanding, Balance	7 years 7 months 20 days	7 years 2 months 12 days	7 years 3 days	7 years 6 months 7 days
Weighted - Average Contractual Life, Granted	9 years 8 months 1 day	9 years 4 months 13 days	9 years 6 months
Weighted - Average Contractual Life, Forefeited		9 years 9 months 29 days
Weighted - Average Contractual Life, Options exercisable	5 years 10 months 24 days	5 years 9 months 3 days	6 years 1 month 6 days
Weighted - Average Contractual Life, Expected to vest	9 years 4 months 6 days	8 years 10 months 6 days	8 years 5 months 15 days
Aggregate Intrinsic Value Outstanding, Beginning balance	$ 243,000	$ 2,870,775	$ 578,128
Aggregate Intrinsic Value, Granted	41,878,351		65,000
Aggregate Intrinsic Value, Exercised	852,818		1,031
Aggregate Intrinsic Value, Forfeited	7,066,915		128,250
Aggregate Intrinsic Value, Outstanding, Ending balance	107,534,863	243,000	2,870,775	$ 578,128
Aggregate Intrinsic Value, Options exercisable	56,918,296	$ 243,000	2,058,419
Aggregate Intrinsic Value, Expected to vest	$ 50,616,567		$ 812,365

Equity Incentive Plans - Summary of Share-based Compensation Expense Recognized (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 5,418,710	$ 1,078,988	$ 545,701
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	2,678,956	377,776	171,388
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 2,739,754	$ 701,212	$ 374,313

Equity Incentive Plans - Summary of Assumption Used to Estimate the Grant Date Fair Value (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Risk-free interest rate, minimum	1.21%	1.21%	1.61%
Risk-free interest rate, maximum	2.30%	1.60%	1.87%
Dividend yield	0.00%	0.00%	0.00%
Volatility, minimum	51.60%	52.00%	78.10%
Volatility, maximum	55.80%	78.10%	93.50%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Expected terms (years)	5 years 3 months 18 days	5 years 2 months 12 days	5 years 4 months 24 days
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Expected terms (years)	7 years	5 years 8 months 12 days	6 years 2 months 12 days

Net Loss per Common Share - Summary of Basic and Diluted Net Loss per Common Share (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net loss and comprehensive loss	$ (18,255,896)	$ (11,573,273)	$ (12,126,152)	$ (9,050,773)	$ (6,620,102)	$ (7,210,591)	$ (7,294,245)	$ (5,999,699)	$ (51,006,094)	$ (27,124,637)	$ (46,515,956)
Denominator:
Weighted-average number of common shares used in net loss per common share, basic and diluted									13,870,949	8,428,366	5,795,040
Net loss per common share, basic and diluted	$ (3.68)	$ (1.37)	$ (0.67)	$ (0.50)	$ (3.22)	$ (0.86)	$ (0.41)	$ (0.33)	$ (3.68)	$ (3.22)	$ (8.03)

Net Loss per Common Share - Summary of Shares Outstanding that were Excluded from Calculation of Diluted Net Loss Per Share (Detail) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount	6,369,645	30,248,133	25,743,322
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	23,269,107
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	6,355,581	3,831,237	2,474,215
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

Selected Quarterly Financial Data (Unaudited) - Summary of Interim Financial Information on Quarterly Basis (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Net Operating Loss	$ (17,568,845)	$ (11,589,319)	$ (12,394,490)	$ (9,214,375)	$ (6,247,048)	$ (7,234,204)	$ (7,572,638)	$ (6,228,452)	$ (50,767,029)	$ (27,282,342)	$ (46,573,093)
Net loss	$ (18,255,896)	$ (11,573,273)	$ (12,126,152)	$ (9,050,773)	$ (6,620,102)	$ (7,210,591)	$ (7,294,245)	$ (5,999,699)	$ (51,006,094)	$ (27,124,637)	$ (46,515,956)
Net loss per common share, basic and diluted	$ (3.68)	$ (1.37)	$ (0.67)	$ (0.50)	$ (3.22)	$ (0.86)	$ (0.41)	$ (0.33)	$ (3.68)	$ (3.22)	$ (8.03)

Subsequent Events - Additional Information (Detail) - $ / shares	1 Months Ended	12 Months Ended
	Feb. 28, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Subsequent Event [Line Items]
Number of shares granted		2,940,444	1,380,463	210,970
Subsequent Event [Member]
Subsequent Event [Line Items]
Number of shares granted	1,126,585
Exercise price range, lower range limit	$ 14.64
Exercise price range, higher range limit	$ 16.45