Apellis Pharmaceuticals (APLS) 10-Q 29 Apr 20 2020 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2020	Apr. 24, 2020
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1
Entity Registrant Name	APELLIS PHARMACEUTICALS, INC.
Entity Central Index Key	0001492422
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity File Number	001-38276
Entity Tax Identification Number	27-1537290
Entity Address, Address Line One	100 Fifth Avenue
Entity Address, City or Town	Waltham
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02451
City Area Code	(617)
Local Phone Number	977-5700
Entity Common Stock, Shares Outstanding		75,513,064
Entity Incorporation, State or Country Code	DE
Entity Interactive Data Current	Yes
Document Quarterly Report	true
Document Transition Report	false
Security 12b Title	Common Stock, $0.0001 par value per share
Trading Symbol	APLS
Security Exchange Name	NASDAQ

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 417,881	$ 351,985
Marketable securities	228,813
Prepaid assets	22,806	19,802
Restricted cash	1,016
Other current assets	1,891	1,308
Total current assets	672,407	373,095
Non-current Assets:
Right-of-use assets	13,715	14,110
Property and equipment, net	1,972	1,655
Other assets	845	385
Total assets	688,939	389,245
Current liabilities:
Accounts payable	3,078	8,361
Accrued expenses	44,930	54,783
Current portion of right of use liabilities	2,732	2,609
Total current liabilities	50,740	65,753
Long-term liabilities:
Convertible senior notes	144,561	142,567
Development derivative liability	223,245	134,839
Operating lease liabilities	11,396	11,857
Total liabilities	429,942	355,016
Stockholders' equity:
Preferred stock, $0.0001 par value; 10.0 million shares authorized, and zero shares issued and outstanding at March 31, 2020 and December 31, 2019	0	0
Common stock, $0.0001 par value; 200.0 million shares authorized at March 31, 2020 and December 31, 2019; 75.4 million shares issued and outstanding at March 31, 2020, and 63.9 million shares issued and outstanding at December 31, 2019	7	6
Additional paid in capital	1,008,275	615,850
Accumulated other comprehensive income/(loss)	1,010	(154)
Accumulated deficit	(750,295)	(581,473)
Total stockholders' equity	258,997	34,229
Total liabilities and stockholders' equity	$ 688,939	$ 389,245

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2020	Dec. 31, 2019
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, issued	0	0
Preferred stock, outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, issued	75,400,000	63,900,000
Common stock, outstanding	75,400,000	63,900,000

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Operating expenses:
Research and development	$ 69,282	$ 40,480
General and administrative	29,504	8,171
Operating loss	(98,786)	(48,651)
Loss on extinguishment of debt		(1,208)
Loss from remeasurement of development derivative liability	(68,406)	(736)
Interest income	2,275	867
Interest expense	(3,919)	(593)
Other income/(expense), net	14	(253)
Net loss	(168,822)	(50,574)
Other comprehensive gain:
Unrealized gain on marketable securities	1,394
Foreign currency gain/ (loss)	(230)	2
Total other comprehensive gain/ (loss)	1,164	2
Comprehensive loss, net of tax	$ (167,658)	$ (50,572)
Net loss per common share, basic and diluted	$ (2.29)	$ (0.87)
Weighted-average number of common shares used in net loss per common share, basic and diluted	73,720	57,897

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Changes in Stockholder's Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2018	$ 160,972	$ 5	$ 437,856	$ (123)	$ (276,766)
Beginning balance, Shares at Dec. 31, 2018		56,279
Issuance of common stock in follow-on offering, net of offering costs	109,604	$ 1	109,603
Issuance of common stock in follow-on offering, shares		6,900
Issuance of common stock upon exercise of stock options	192		192
Issuance of common stock upon exercise of stock options, shares		39
Share-based compensation expense	4,559		4,559
Net loss	(50,574)				(50,574)
Foreign currency gain (loss)	2			2
Ending balance at Mar. 31, 2019	224,755	$ 6	552,210	(121)	(327,340)
Ending balance, Shares at Mar. 31, 2019		63,218
Beginning balance at Dec. 31, 2019	34,229	$ 6	615,850	(154)	(581,473)
Beginning balance, Shares at Dec. 31, 2019		63,938
Issuance of common stock in follow-on offering, net of offering costs	381,458	$ 1	381,457
Issuance of common stock in follow-on offering, shares		10,925
Issuance of common stock upon exercise of stock options	1,674		1,674
Issuance of common stock upon exercise of stock options, shares		559
Share-based compensation expense	9,294		9,294
Unrealized gain on available-for-sale investments	1,394			1,394
Net loss	(168,822)				(168,822)
Foreign currency gain (loss)	(230)			(230)
Ending balance at Mar. 31, 2020	$ 258,997	$ 7	$ 1,008,275	$ 1,010	$ (750,295)
Ending balance, Shares at Mar. 31, 2020		75,422

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Operating Activities
Net loss	$ (168,822)	$ (50,574)
Adjustments to reconcile net loss to net cash used in operating activities:
Share-based compensation expense	9,294	4,559
Loss on early extinguishment of debt		1,208
Loss from remeasurement of development derivative liability	68,406	736
Amortization of right-of-use assets	57	73
Depreciation expense	85	41
Amortization of debt discounts		19
Amortization of term loan facility discounts		104
Amortization of discounts for convertible notes, net of financing costs	1,994
Changes in operating assets and liabilities:
Prepaid assets	(3,004)	3,824
Other current assets	(671)	(250)
Other assets	(460)	(46)
Accounts payable	(5,237)	1,906
Accrued expenses	(9,991)	2,764
Other liabilities		76
Net cash used in operating activities	(108,349)	(35,560)
Investing Activities
Purchase of property and equipment	(435)	(875)
Purchase of available-for-sale securities	(227,419)
Net cash used in investing activities	(227,854)	(875)
Financing Activities
Proceeds from issuance of common stock, net of issuance costs	381,593	109,927
Proceeds from development derivative liability	20,000	60,000
Proceeds from exercise of stock options	1,674	192
Repayment of term loan facility		(21,701)
Net cash provided by financing activities	403,267	148,418
Effect of exchange rate changes on cash, cash equivalents and restricted cash	(152)	(4)
Net increase in cash, cash equivalents and restricted cash	66,912	111,979
Cash, cash equivalents and restricted cash at beginning of period	351,985	176,268
Cash, cash equivalents and restricted cash at end of period	418,897	288,247
Reconciliation of cash, cash equivalents and restricted cash to the consolidated balance sheets:
Cash and cash equivalents	417,881	288,247
Restricted cash	1,016
Cash, cash equivalents and restricted cash at end of period	418,897	288,247
Supplemental Disclosure of Financing Activities
Cash paid for Interest	$ 3,829	$ 588

Nature of Organization and Oper

Nature of Organization and Operations	3 Months Ended
	Mar. 31, 2020
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of Organization and Operations	1. Nature of Organization and Operations Apellis Pharmaceuticals, Inc. (the “Company”) is a clinical-stage biopharmaceutical company focused on the development of novel therapeutic compounds to treat disease through the inhibition of the complement system, which is an integral component of the immune system, at the level of C3, the central protein in the complement cascade. The Company was incorporated in September 2009 under the laws of the State of Delaware and has its principal office in Waltham, Massachusetts. 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”). Additionally, the Company is subject to risks arising from the Coronavirus Disease 2019 (COVID-19) pandemic, which could have adverse effects upon our business and operations, including on our ability to initiate, conduct and complete clinical trials. Follow-on Public Offerings On January 13, 2020, the Company issued and sold 10,925 shares of its common stock at a price per share to the public of $37.00 in a follow-on public offering including an additional 1,425 shares of its common stock that were sold at the follow-on public offering price of $37.00 per share pursuant to the underwriters’ in full exercise of their option to purchase additional shares of common stock. The Company received net proceeds of $381.5 million after deducting underwriting discounts and commissions of $22.2 million and offering costs of $0.5 million for these transactions. Liquidity and Financial Condition The accompanying unaudited condensed consolidated financial statements have been prepared on the basis of the realization of assets and the satisfaction of liabilities and commitments in the normal course of business. As of April 29, 2020, the date of issuance of these unaudited condensed consolidated financial statements, the Company believes that its cash and cash equivalents of $417.9 million and marketable securities of $228.8 million as of March 31, 2020 will be sufficient to fund its operations and capital expenditure requirements for at least the next twelve months from the date of issuance of the unaudited interim consolidated financial statements. The future viability of the Company beyond that point is dependent on its ability to raise additional capital to finance its operations. The Company is subject to risks common to other life science companies in the development stage including, but not limited to, uncertainty of product development and commercialization, lack of marketing and sales history, development by its competitors of new technological innovations, dependence on key personnel, market acceptance of products, product liability, protection of proprietary technology, ability to raise additional financing, and compliance with FDA and other government regulations. If the Company does not successfully commercialize any of its product candidates, it will be unable to generate recurring product revenue or achieve profitability. Management’s plans in order to meet its short-term and longer-term operating cash flow requirements include obtaining additional funding. There are uncertainties associated with the Company’s ability to (1) obtain additional debt or equity financing on terms that are favorable to the Company (2) enter into collaborative agreements with strategic partners, and (3) succeed in its future operations. If the Company is not able to obtain the required funding for its operations, or is not able to obtain such funding on terms that are favorable to the Company, it could be forced to delay, reduce or eliminate its research and development programs or future commercialization efforts and its business could be materially harmed.

Basis of Presentation

Basis of Presentation	3 Months Ended
	Mar. 31, 2020
Accounting Policies [Abstract]
Basis of Presentation	2. Basis of Presentation The accompanying unaudited condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries, Apellis Australia Pty Ltd, Apellis Ireland Ltd, Apellis Switzerland GmbH and Apellis MA Securities Corp. All intercompany balances and transactions have been eliminated in consolidation. The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“U.S. GAAP”) and following the requirements of the Securities and Exchange Commission (the “SEC”), for interim reporting. As permitted under those rules, certain footnotes or other financial information that are normally required by U.S. GAAP have been condensed or omitted and, accordingly, the balance sheet as of December 31, 2019 has been derived from audited consolidated financial statements at that date but does not include all of the information required by U.S. GAAP for complete financial statements. These financial statements have been prepared on the same basis as the Company’s annual financial statements and, in the opinion of management, reflect all adjustments (consisting only of normal recurring adjustments) that are necessary for a fair presentation of the Company’s financial information. The results of operations for the three months ended March 31, 2020 are not necessarily indicative of the results to be expected for the year ending December 31, 2020 or for any other interim period or for any other future year. The accompanying unaudited condensed consolidated financial statements and related financial information should be read in conjunction with the audited consolidated financial statements and the related notes thereto for the year ended December 31, 2019 included in the Company’s Annual Report on Form 10-K filed with the Securities and Exchange Commission on February 27, 2020. Fair Value of Financial Instruments The Company is required to disclose information on the fair value of financial instruments and inputs that enable an assessment of the fair value. The three levels of the fair value hierarchy prioritize valuation inputs based upon the observable nature of those inputs as follows: Level 1 – Quoted prices in active markets for identical assets or liabilities; Level 2 – Inputs other than quoted prices included within level 1 that are observable for the asset or liability, either directly or indirectly; Level 3 – Unobservable inputs that reflect the Company’s own assumptions about the assumptions market participants would use in pricing the asset or liability. The Company’s financial instruments, in addition to those presented in Note 5, Long-term Debt, Note 7, Marketable Securities, and Note 9, Fair Value Measurements, include cash and cash equivalents, the Australian research and development credit, accounts payable and accrued liabilities. Management believes that the carrying amounts of certain cash and cash equivalents, the Australian research and development credit, accounts payable and accrued expenses approximate the fair value due to the short-term nature of those instruments. Cash and Cash Equivalents Cash and cash equivalents 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 certain cash and cash equivalents are valued at cost, which approximates their fair value. See Note 9, Fair Value Measurements, for additional information. Restricted Cash The Company is contingently liable under unused letters of credit with a bank, related to the Company’s customs import bond and facility lease agreements of $1.0 million as of March 31, 2020. The Company records as restricted cash the collateral used to secure these letters of credit. The Company did not have restricted cash as of December 31, 2019. Foreign Currency The financial position and results of operations of the Company's Australian, Swiss and Irish subsidiaries are measured using the foreign subsidiary's local currency. Revenues and expenses of the subsidiaries have been translated into U.S. dollars at average exchange rates prevailing during the respective periods. Assets and liabilities have been translated at the rates of exchange on the balance sheet date. The resulting translation gain and loss adjustments are recorded directly as a separate component of stockholders’ equity .

Accrued Expenses

Accrued Expenses	3 Months Ended
	Mar. 31, 2020
Text Block [Abstract]
Accrued Expenses	3. Accrued Expenses Accrued expenses are as follows: March 31, December 31, 2020 2019 Accrued research and development $ 30,030 $ 36,449 Accrued payroll liabilities 7,898 11,443 Other 7,002 6,891 Total $ 44,930 $ 54,783

Development Derivative Liabilit

Development Derivative Liability	3 Months Ended
	Mar. 31, 2020
Research And Development [Abstract]
Development Derivative Liability	4. Development Derivative Liability On February 28, 2019, the Company entered into a development funding agreement, which we refer to as the SFJ Agreement, with SFJ Pharmaceuticals Group or SFJ, under which SFJ agreed to provide funding to the Company to support the development of pegcetacoplan for the treatment of patients with PNH. Pursuant to the agreement, SFJ paid the Company $60.0 million following the signing of the agreement, and agreed to pay the Company up to an additional $60.0 million in the aggregate in three equal installments upon the achievement of specified development milestones with respect to the Company’s Phase 3 program for pegcetacoplan in PNH and subject to the Company having cash resources at the time sufficient to fund at least 10 months of the Company’s operations. In addition, upon the mutual agreement of the Company and SFJ, at any time after the earlier of the date that the Company has reviewed the primary endpoint data from its PEGASUS Phase 3 trial of pegcetacoplan in patients with PNH and March 31, 2020, SFJ may fund an additional $50.0 million of the Company’s development costs which we refer to as the Additional SFJ Funding. On June 7, 2019, the Company and SFJ amended the development funding agreement, which we refer to as the SFJ Amendment. Under the SFJ Amendment, SFJ agreed to make an additional $20.0 million funding payment to the Company to support the development of pegcetacoplan for the treatment of patients with PNH. This additional $20.0 million payment is in addition to and not part of the Additional SFJ Funding. On June 27, 2019, the Company received $40.0 million from SFJ, consisting of $20.0 million as the first installment of the additional $60.0 million upon the achievement of a milestone and the $20.0 million payable under the SFJ Amendment. On September 23, 2019, the Company received $20.0 million from SFJ as the second installment of the additional $60.0 million upon the achievement of a milestone. The Company met the remaining development milestone under the SFJ Agreement upon the Company’s announcement of the primary endpoint data from its PEGASUS Phase 3 trial on January 7, 2020 and the final installment of $20.0 million of the additional $60.0 million was paid on January 29, 2020. Under the SFJ Agreement following regulatory approval by the FDA or EMA for the use of pegcetacoplan as a treatment for PNH the Company will be obligated to pay SFJ an initial payment of up to $5.0 million (or a total of $10.0 million if regulatory approval is granted by the FDA and the EMA) and then up to an additional $226.0 million in the aggregate (or up to $452.0 million if regulatory approval is granted by the FDA and the EMA) in six additional annual payments with the majority of the payments being made from the third anniversary to the sixth anniversary of regulatory approval. Such payments will be proportionately adjusted in the event that the actual funding from SFJ is greater than $120.0 million (including as a result of the payment of the Additional SFJ Funding but excluding the $20.0 million funding payment made under the SFJ Amendment). Additionally, the Company granted a security interest in all of its assets, excluding intellectual property and license agreements to which it is a party. In connection with the grant of the security interest, the Company agreed to certain affirmative and negative covenants, including restrictions on its ability to pay dividends, incur additional debt or enter into licensing transactions with respect to its intellectual property, other than specified types of licenses. The SFJ Agreement is presented as a derivative liability on the balance sheet. The liability was initially recorded at the value of the $60.0 million aggregate cash received pursuant to the contractual terms, which was determined to have been fair value, and subsequently remeasured at March 31, 2019 as a level 3 derivative, with the change in fair value of $0.7 million recorded for the three months ended March 31, 2019 in loss from remeasurement of development derivative liability on the statement of operations. During 2019, the Company received an additional $60.0 million under the SFJ Agreement and the Company met additional milestones in the agreement. The remeasurement of the development derivative liability at December 31, 2019 resulted in a remeasured fair value of $ million on the consolidated balance sheet a t December 31, 2019 . The Company received an additional $ million in January 2020 and the derivative liability under the SFJ Agreement (as amended) , was subsequently remeasured at March 31, 2020 as a level 3 derivative, with the change in fair value of $ million recorded for the three months ended March 31 , 20 20 in the loss from remeasurement of development derivative liability on the income statement , with a remeasured fair value of $ million on the consolidated balance sheet at March 31, 2020 . The following table presents a rollforward of the development derivative liability: Three Months Ended March 31, 2020 2019 Balance at fair market value, January 1, $ 134,839 $ — Amounts received under the SFJ agreement and SFJ amendment 20,000 60,000 Loss recorded in loss from remeasurement of development derivative liability 68,406 736 Balance at fair market value, March 31, $ 223,245 $ 60,736 The derivative is valued using a scenario-based discounted cash flow method, whereby each scenario makes assumptions about the probability and timing of cash flows, and such cash flows are present valued using a risk-adjusted discount rate. The analysis is calibrated such that the value of the derivative as of the date of the SFJ Agreement was consistent with an arm’s-length transaction. Key inputs to the level 3 fair value model include (i) the probability and timing of achieving stated development milestones to receive the next tranches of funding, (ii) the probability and timing of achieving FDA and EMA approval, (iii) SFJ’s cost of borrowing (8.0%), and (iv) the Company’s cost of borrowing (13.47%). SFJ’s implied cost of borrowing was 8.0% and the Company’s implied cost of borrowing was 13.47% as of the reporting date. These implied costs of borrowing were determined assuming the SFJ Agreement was initially executed with arm’s-length terms. If the SFJ Agreement was instead not determined to be an arm’s-length transaction, then implied discount rates could differ.

Long-term Debt

Long-term Debt	3 Months Ended
	Mar. 31, 2020
Debt Disclosure [Abstract]
Long-term Debt	5. Long-term Debt Convertible Senior Notes On September 16, 2019, the Company completed a private offering of Convertible Notes with an aggregate principal amount of $220.0 million which were issued pursuant to an indenture (the “Indenture”) with U.S. Bank National Association, as trustee. The net proceeds from the sale of the Convertible Notes were approximately $212.9 million after deducting the initial purchasers’ discounts and commissions of $6.6 million and offering expenses of $0.5 million by the Company. The Company used $28.4 million of the net proceeds from the sale to pay the cost of the capped call transactions described below. The Convertible Notes are senior unsecured obligations of the Company and bear interest at a rate of 3.5% per year payable semiannually in arrears on March 15 and September 15 of each year, beginning on March 15, 2020. The Convertible Notes will mature on September 15, 2026, unless converted earlier, redeemed or repurchased in accordance with their terms. The Convertible Notes are convertible into shares of the Company’s common stock at an initial conversion rate of 25.3405 shares per $1,000 principal amount of Convertible Notes (equivalent to an initial conversion price of approximately $39.46 per share of common stock). The conversion rate is subject to customary anti-dilution adjustments. In addition, following certain events that occur prior to the maturity date or if the Company deliver a notice of redemption, the Company will increase the conversion rate for a holder who elects to convert its Convertible Notes in connection with such corporate event or a notice of redemption, as the case may be, in certain circumstances as provided in the indenture. Prior to March 15, 2026, the Convertible Notes are convertible only upon the occurrence of certain events. On or after March 15, 2026 until the close of business on the second scheduled trading day immediately preceding the maturity date of the Convertible Notes, holders may convert the Convertible Notes at any time. Upon conversion of the Convertible Notes, the Company will pay or deliver, as the case may be, cash, shares of the Company’s common stock or a combination of cash and shares of common stock, at the Company’s election. Prior to September 20, 2023, the Company may not redeem the Convertible Notes. The Company may redeem for cash all or a portion of the Convertible Notes, at its option, on or after September 20, 2023 if the last reported sale price of the Company’s common stock has been at least 130% of the conversion price then in effect for at least 20 trading days (whether or not consecutive), including the trading day immediately preceding the date on which the Company provides a notice of redemption, during any 30 consecutive trading day period ending on, and including, the trading day immediately preceding the date on which the Company provides notice of redemption. The redemption price will be equal to 100% of the principal amount of the Convertible Notes to be redeemed, plus accrued and unpaid interest to, but excluding, the redemption date. If the Company undergoes a “fundamental change,” as defined in the Indenture, prior to maturity, subject to certain conditions, holders may require the Company to repurchase for cash all or any portion of their Convertible Notes at a fundamental change repurchase price equal to 100% of the principal amount of the notes to be repurchased, plus any accrued and unpaid interest to, but excluding, the fundamental change repurchase date. T he Company used an effective interest rate of 10.5% to determine the liability component of the Convertible Notes. This resulted in the recognition of $145.1 million as the liability component of the Convertible Notes and the recognition of the residual amount of $74.9 million as the debt discount with a corresponding increase to additional paid in capital for the equity component of the Convertible Notes. Aggregate debt issuance costs of $7.1 million was allocated to the liability and equity components in the amounts of $4.7 million and $2.4 million, respectively. Interest expense for the Convertible Notes was $3.9 million for the three months ended March 31, 2020 which included the amortization of the discount on the Convertible Notes of $1.9 million, accrued semi-annual coupon payable of $1.9 million and amortization of debt issuance costs of $0.1 million. As of March 31, 2020, $0.7 million of debt issuance costs was recorded on the consolidated balance sheet as a reduction to the carrying amount of the Convertible Notes. The aggregate balance of the Convertible Notes, net of unamortized debt issuance costs, as of March 31, 2020 and December 31, 2019 was $144.6 million and $142.6 million respectively. Capped Call Transactions On September 11, 2019, concurrently with the pricing of the Convertible Notes, the Company entered into capped call transactions with two counterparties. The capped call transactions are expected generally to reduce the potential dilution to the Company’s common stock upon any conversion of Convertible Notes and/or offset any cash payments the Company is required to make in excess of the principal amount of converted Convertible Notes, as the case may be, in the event that the market price per share of the Company’s common stock, as measured under the terms of the capped call transactions, is greater than the strike price of the capped call transactions, which is initially $39.4625 (the conversion price of the Convertible Notes) and is subject to anti-dilution adjustments substantially similar to those applicable to the conversion rate of such Convertible Notes. If, however, the market price per share of the Company’s common stock, as measured under the terms of the capped call transactions, exceeds the cap price of the capped call transactions, which is initially $63.14 per share, representing a premium of 100% above the last reported sale price of $31.57 per share of its common stock on The Nasdaq Global Select Market on September 11, 2019, there would nevertheless be dilution and/or there would not be an offset of such potential cash payments, in each case, to the extent that such market price exceeds the cap price of the capped call transactions. Pursuant to ASC 815-40 Derivatives and Hedging Term Loan Facility On October 20, 2017, the Company entered into a loan and security agreement with Silicon Valley Bank (“SVB”) to provide for a $20.0 million term loan facility (the “term loan facility”). Borrowings under the term loan facility accrued interest at a floating rate per annum equal to the WSJ prime rate plus 1.50%. Under the agreement, the Company was required to make monthly interest-only payments through November 1, 2019 and was required to make 24 equal monthly payments of principal, plus accrued interest, from November 1, 2019 through October 1, 2021, when all unpaid principal and interest became due and payable. On March 26, 2019, the Company voluntarily repaid all outstanding amounts due and owed, including applicable termination fees, under the term loan facility. The final payment of $21.8 million totaled per diem interest of $0.1 million and $21.7 million for the outstanding balance of the term loan which included (i) a final payment equal to 8% of the original principal amount of the term loan o f $ million , and ( i i) a prepayment fee contractually owed of $ 0.1 million plus other fees which resulted in a total loss on extinguishment of debt of $ million . In connection with the Company’s entry into the term loan facility, the Company issued to SVB a warrant to purchase 14,064 shares of the Company’s common stock with an exercise price per share of $5.484. The warrant has a ten-year Promissory Note On October 19, 2017, the Company issued and sold an unsecured promissory note in the principal amount of $7.0 million to Golda Darty Partners S.A. (“GDP”). The promissory note accrued interest at a rate per annum of 8.0%, and was due and payable quarterly in arrears on the 19th day of each April, July, October and January. The promissory note had a maturity date of October 19, 2022 when the $7.0 million would be due and payable in its entirety. The promissory note was contractually subordinated to the Company’s obligations to SFJ under the SFJ Agreement. On September 16, 2019, the Company voluntarily repaid all outstanding amounts due and owed under the promissory note, except for a small amount of interest subsequently repaid. The payment of $7.1 million totaled per diem interest of $0.1 million and $7.0 million for the outstanding principal balance of the promissory note. In connection with the issuance and sale of the above promissory note, the Company issued to GDP a warrant to purchase 93,764 shares of the Company’s common stock at a price per share of $5.484, which was exercised in whole in October 2017. The Company recorded the fair value of the warrant in the aggregate amount of $0.4 million as a discount to the promissory note. This amount was being accreted as additional interest expense over the term of the promissory note. Upon the repayment of the promissory note, the Company recorded a loss on extinguishment of debt of $0.3 million due to the remaining discount. The contractual maturities of the Company’s long-term debt obligations due subsequent to March 31, 2020 consist of the $220.0 million Convertible Senior Notes which mature in September 2026.

Leases

Leases	3 Months Ended
	Mar. 31, 2020
Leases [Abstract]
Leases	6. Leases On January 1, 2019, The Company adopted ASU 2016-02 Leases (Topic 842) As a practical expedient permitted under Topic 842, the Company elected to account for the lease and non-lease components as a single lease component for all leases of which it is the lessee. Lease payments, which may include lease and non-lease components, are included in the measurement of the Company’s lease liabilities to the extent that such payments are either fixed amounts or variable amounts that depend on a rate or index as stipulated in the lease contract. When the Company cannot readily determine the rate implicit in the lease, the Company determines its incremental borrowing rate by using the rate of interest that it would have to pay to borrow on a collateralized basis over a similar term, an amount equal to the lease payments in a similar economic environment. The Company enters into lease agreements with terms generally ranging from 2-7 years. Some of the Company’s lease agreements include Company options to extend the lease on a month to month basis or for set periods for up to five years. options to terminate the leases within one year or per other contractual terms. As of March 31, 2020 and December 31, 2019, all leases were classified as operating lease assets and liabilities. Additional information related to the operating lease assets and liabilities is as follows: March 31, December 31, 2020 2019 Operating Lease Assets $ 13,715 $ 14,110 Operating Lease Liabilities $ 14,128 $ 14,466 Weighted Average Remaining Term in years 4.77 5.01 Weighted Average discount rate used to measure outstanding lease liabilities 8.32 % 8.19 % For the three months ended March 31, 2020 and 2019, the total lease cost for operating lease expense was: $1.0 million and $0.4 million, respectively. Supplemental cash flow information related to operating leases for the three months ended March 31 is as follows: 2020 2019 Operating cash flows for operating leases $ 968 $ 331 Operating lease assets obtained in exchange for lease obligations $ 334 $ 6,940 The maturities of the Company’s operating lease liabilities as of March 31, 2020 are as follows: 2020 $ 2,883 2021 3,518 2022 3,283 2023 3,288 2024 and thereafter 4,107 Total future minimum lease payments less 17,079 Imputed interest (2,951 ) Total operating lease liabilities $ 14,128

Marketable Securities

Marketable Securities	3 Months Ended
	Mar. 31, 2020
Investments Debt And Equity Securities [Abstract]
Marketable Securities	7. Marketable Securities The amortized cost, gross unrealized holding losses and fair value of available-for-sale debt securities by type of security as of March 31, 2020 were as follows: For the Three Months Ended March 31, 2020 Amortized Cost Gross Unrealized Holding Gains Gross Unrealized Holding Losses Fair Value U.S. Government-related obligations $ 227,419 $ 1,394 $ — $ 228,813 All available-for-sale securities mature in one year or less. The Company did not hold available-for-sale securities as of December 31, 2019.

Other Comprehensive Income and

Other Comprehensive Income and Accumulated Other Comprehensive Income	3 Months Ended
	Mar. 31, 2020
Equity [Abstract]
Other Comprehensive Income and Accumulated Other Comprehensive Income	8. Other Comprehensive Income and Accumulated Other Comprehensive Income The following tables summarize the changes in accumulated other comprehensive income/(loss), by component for the three months ended March 31, 2020 and 2019: Unrealized Gains (Losses) from Marketable Securities Foreign Currency Translation Adjustment Total Accumulated Other Comprehensive Income (Loss) Balances, December 31, 2019 $ — $ (154 ) $ (154 ) Net other comprehensive income (loss) 1,394 (230 ) 1,164 Balances, March 31, 2020 $ 1,394 $ (384 ) $ 1,010 Unrealized Gains (Losses) from Marketable Securities Foreign Currency Translation Adjustment Total Accumulated Other Comprehensive Income (Loss) Balances, December 31, 2018 $ — $ (123 ) $ (123 ) Net other comprehensive income (loss) — 2 2 Balances, March 31, 2019 $ — $ (121 ) $ (121 )

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2020
Fair Value Disclosures [Abstract]
Fair Value Measurements	9 . Fair Value Measurements The Company is required to disclose information on the fair value of financial instruments and inputs that enable an assessment of the fair value. The three levels of the fair value hierarchy prioritize valuation inputs based upon the observable nature of those inputs as follows: Level 1 – Quoted prices in active markets for identical assets or liabilities; Level 2 – Inputs other than quoted prices included within level 1 that are observable for the asset or liability, either directly or indirectly; Level 3 – Unobservable inputs that reflect the Company’s own assumptions about the assumptions market participants would use in pricing the asset or liability. The following table presents the fair value of financial instruments recorded originally at amortized cost or fair value and not remeasured on a recurring basis: March 31, 2020 Balance Sheet Classification Type of Instrument Level 1 Level 2 Level 3 Total Financial Assets: Cash and cash equivalents: Money market funds $ 256,106 $ — $ — $ 256,106 Bank certificates of deposit 40,811 — — 40,811 Total Financial Assets $ 296,917 $ — $ — $ 296,917 Financial Liabilities: Convertible senior notes Convertible senior notes* $ — $ — $ 155,008 $ 155,008 Total Financial Liabilities $ — $ — $ 155,008 $ 155,008 December 31, 2019 Balance Sheet Classification Type of Instrument Level 1 Level 2 Level 3 Total Financial Assets: Cash and cash equivalents Money market funds $ 281,314 $ — $ — $ 281,314 Total Financial Assets $ 281,314 $ — $ — $ 281,314 Financial Liabilities: Convertible senior notes Convertible senior notes* $ — $ — $ 149,496 $ 149,496 Total Financial Liabilities $ — $ — $ 149,496 $ 149,496 * The convertible notes were measured and recognized at fair value on the consolidated balance sheet at inception. The fair value of debt component of the Company's Convertible Notes was $155.0 million and $149.5 million, as of March 31, 2020 and December 31, 2019, respectively, based on the discounted cash flows for comparable straight debt instrument. The Convertible Notes accrue a semi-annual coupon at an annual rate of 3.5%, which was included in accrued expenses in the consolidated balance sheets at March 31, 2020 and December 31, 2019. The following table presents the fair value of financial instruments recorded at fair value at inception and remeasured on a recurring basis: March 31, 2020 Balance Sheet Classification Type of Instrument Level 1 Level 2 Level 3 Total Financial Assets: Cash and cash equivalents: US government obligations $ 117,184 $ — $ — $ 117,184 Marketable Securities: US government obligations 228,813 — — 228,813 Total Financial Assets $ 345,997 $ — $ — $ 345,997 Financial Liabilities: Development derivative liability Development derivative liability $ — $ — $ 223,245 $ 223,245 Total Financial Liabilities $ — $ — $ 223,245 $ 223,245 December 31, 2019 Balance Sheet Classification Type of Instrument Level 1 Level 2 Level 3 Total Financial Liabilities: Development derivative liability Development derivative liability $ — $ — $ 134,839 $ 134,839 Total Financial Liabilities $ — $ — $ 134,839 $ 134,839 The fair value of the SFJ Agreement is presented as a derivative liability based on level 3 inputs. The derivative is valued using a scenario-based discounted cash flow method, whereby each scenario makes assumptions about the probability and timing of cash flows, and such cash flows are present valued using a risk-adjusted discount rate. The analysis is calibrated such that the value of the derivative as of the date of the SFJ Agreement was consistent with an arm’s-length transaction. Key inputs to the level 3 fair value model include (i) the probability and timing of achieving stated development milestones to receive the next tranches of funding, (ii) the probability and timing of achieving FDA and EMA approval, (iii) SFJ’s cost of borrowing (8.0%), and (iv) the Company’s cost of borrowing (13.47%). SFJ’s implied cost of borrowing was 8.0% and the Company’s implied cost of borrowing was 13.47% as of the reporting date. These implied costs of borrowing were determined assuming the SFJ Agreement was initially executed with arm’s-length terms. If the SFJ Agreement was instead not determined to be an arm’s-length transaction, then implied discount rates could differ.

Refundable Research and Develop

Refundable Research and Development Credit and Income Taxes	3 Months Ended
	Mar. 31, 2020
Refundable Research And Development Credit And Income Taxes [Abstract]
Refundable Research And Development Credit And Income Taxes	10. Refundable Research and Development Credit and Income Taxes The Company earns non-income related refundable Australian research and development credits that are settled and paid to the Company annually. The associated income from the credits are an offset to research and development expenses. The Company’s income tax provision is computed based on the federal statutory rate and the average state statutory rates, net of the related federal benefit. For the three months ended March 31, 2020 and 2019, there were no current or deferred income tax expenses or benefits due to the Company’s net losses, research and development credits and increases in its deferred tax asset valuation allowance during those periods. The Company’s estimate of the realizability of the deferred tax asset is dependent on estimates of projected future levels of taxable income. In analyzing future taxable income levels, the Company considered all evidence currently available, both positive and negative. Based on this analysis, the Company has recorded a valuation allowance for all deferred tax assets as of March 31, 2020.

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2020
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	11. Commitments and Contingencies The Company has entered into a leasing agreement for lab space in Watertown, Massachusetts with an estimated commencement date of June 2, 2020 (the “Lab Lease”). The Lab Lease has a term of 85 months and includes leasing 9,704 square feet of office space. The Lab Lease provides for initial monthly lease payments of $0.06 million per month. The base rent payable over the lease period is $5.4 million. The Company contracts to conduct research and development activities with third parties. Certain of these contracts commit the Company to pay future milestone payments up to $15.0 million or to pay royalty fees ranging from 3-6% if any of the research results in regulatory approval or commercial revenue for a product. The scope of the services under the research and development contracts can be modified and the contracts cancelled by the Company upon written notice. In some instances, the contracts may be cancelled by the third party upon written notice. If the Company were to cancel these contracts as of March 31, 2020, the Company would be required to pay certain termination costs and other fees of approximately $2.8 million that would be incurred in future periods. As of March 31, 2020, the Company has non-cancellable purchase commitments for 2020 with certain manufacturing vendors in the amount of approximately $22.6 million. The Company also has certain payment and other obligations under the SFJ Agreement, which are discussed above in Note 4.

Net Loss per Share

Net Loss per Share	3 Months Ended
	Mar. 31, 2020
Earnings Per Share [Abstract]
Net Loss per Share	12. Net Loss per Share Since the Company was in a loss position for all periods presented, basic net loss per common share is the same as diluted net loss per common share for all periods presented as the inclusion of all potential common shares outstanding would have been anti-dilutive. Convertible notes and shares outstanding presented below were excluded from the calculation of diluted net loss per share, prior to the use of the treasury stock method, as their effect is anti-dilutive: For the Three Months Ended March 31, 2020 2019 Convertible notes 5,575 — Common stock options 9,434 9,191 Restricted stock units 298 — Common stock warrants — 14 Total 15,307 9,205

Related Party Transaction

Related Party Transaction	3 Months Ended
	Mar. 31, 2020
Related Party Transactions [Abstract]
Related Party Transaction	13. Related Party Transaction Effective as of May 1, 2018, the Company entered into a subscription license agreement and a services agreement with Revon Systems, Inc. (“Revon”). Under the subscription license agreement, Revon granted the Company an exclusive license to use the Revon software platform and applications for any purpose with respect to the Company's programs in age-related macular degeneration, hemolytic diseases and complement-dependent nephropathies for an annual license fee of $0.2 million for a two year initial term and an option to obtain a perpetual, exclusive license thereafter for $0.4 million. Under the services agreement, Revon provided development services with respect to the Revon software to the Company for $0.3 million during the first year. Prior to the acquisition of Revon by an unrelated third party in July 2019, the Company paid the remainder of the annual license fee due for the second year, exercised the option for the perpetual license and discontinued the services agreement. Each of Cedric Francois, the Company’s chief executive officer, Pascal Deschatelets, the Company’s chief operating officer, and Alec Machiels, a member of the board of directors, is an affiliate of Revon. The Board approved the Revon agreements after review by a subcommittee of the disinterested members of the Board and determination by the full Board that the terms of the Revon agreements were fair, reasonable and in the best interests of the Company. The exercise of the option for the perpetual license was approved in accordance with the Company’s related party transaction policy.

Basis of Presentation (Policies

Basis of Presentation (Policies)	3 Months Ended
	Mar. 31, 2020
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries, Apellis Australia Pty Ltd, Apellis Ireland Ltd, Apellis Switzerland GmbH and Apellis MA Securities Corp. All intercompany balances and transactions have been eliminated in consolidation. The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“U.S. GAAP”) and following the requirements of the Securities and Exchange Commission (the “SEC”), for interim reporting. As permitted under those rules, certain footnotes or other financial information that are normally required by U.S. GAAP have been condensed or omitted and, accordingly, the balance sheet as of December 31, 2019 has been derived from audited consolidated financial statements at that date but does not include all of the information required by U.S. GAAP for complete financial statements. These financial statements have been prepared on the same basis as the Company’s annual financial statements and, in the opinion of management, reflect all adjustments (consisting only of normal recurring adjustments) that are necessary for a fair presentation of the Company’s financial information. The results of operations for the three months ended March 31, 2020 are not necessarily indicative of the results to be expected for the year ending December 31, 2020 or for any other interim period or for any other future year. The accompanying unaudited condensed consolidated financial statements and related financial information should be read in conjunction with the audited consolidated financial statements and the related notes thereto for the year ended December 31, 2019 included in the Company’s Annual Report on Form 10-K filed with the Securities and Exchange Commission on February 27, 2020.
Fair Value of Financial Instruments	Fair Value of Financial Instruments The Company is required to disclose information on the fair value of financial instruments and inputs that enable an assessment of the fair value. The three levels of the fair value hierarchy prioritize valuation inputs based upon the observable nature of those inputs as follows: Level 1 – Quoted prices in active markets for identical assets or liabilities; Level 2 – Inputs other than quoted prices included within level 1 that are observable for the asset or liability, either directly or indirectly; Level 3 – Unobservable inputs that reflect the Company’s own assumptions about the assumptions market participants would use in pricing the asset or liability. The Company’s financial instruments, in addition to those presented in Note 5, Long-term Debt, Note 7, Marketable Securities, and Note 9, Fair Value Measurements, include cash and cash equivalents, the Australian research and development credit, accounts payable and accrued liabilities. Management believes that the carrying amounts of certain cash and cash equivalents, the Australian research and development credit, accounts payable and accrued expenses approximate the fair value due to the short-term nature of those instruments.
Cash and Cash Equivalents	Cash and Cash Equivalents Cash and cash equivalents are defined as cash in banks and investment instruments having maturities of three months or less from their acquisition date. The carrying amounts reported in the consolidated balance sheets for certain cash and cash equivalents are valued at cost, which approximates their fair value. See Note 9, Fair Value Measurements, for additional information.
Restricted Cash	Restricted Cash The Company is contingently liable under unused letters of credit with a bank, related to the Company’s customs import bond and facility lease agreements of $1.0 million as of March 31, 2020. The Company records as restricted cash the collateral used to secure these letters of credit. The Company did not have restricted cash as of December 31, 2019.
Foreign Currency	Foreign Currency The financial position and results of operations of the Company's Australian, Swiss and Irish subsidiaries are measured using the foreign subsidiary's local currency. Revenues and expenses of the subsidiaries have been translated into U.S. dollars at average exchange rates prevailing during the respective periods. Assets and liabilities have been translated at the rates of exchange on the balance sheet date. The resulting translation gain and loss adjustments are recorded directly as a separate component of stockholders’ equity .

Accrued Expenses (Tables)

Accrued Expenses (Tables)	3 Months Ended
	Mar. 31, 2020
Text Block [Abstract]
Schedule of Accrued Expenses	Accrued expenses are as follows: March 31, December 31, 2020 2019 Accrued research and development $ 30,030 $ 36,449 Accrued payroll liabilities 7,898 11,443 Other 7,002 6,891 Total $ 44,930 $ 54,783

Development Derivative Liabil_2

Development Derivative Liability (Tables)	3 Months Ended
	Mar. 31, 2020
Research And Development [Abstract]
Schedule of Development Derivative Liability	The following table presents a rollforward of the development derivative liability: Three Months Ended March 31, 2020 2019 Balance at fair market value, January 1, $ 134,839 $ — Amounts received under the SFJ agreement and SFJ amendment 20,000 60,000 Loss recorded in loss from remeasurement of development derivative liability 68,406 736 Balance at fair market value, March 31, $ 223,245 $ 60,736

Leases (Tables)

Leases (Tables)	3 Months Ended
	Mar. 31, 2020
Leases [Abstract]
Schedule of Additional Information Related to Operating Lease Assets and Liabilities	Additional information related to the operating lease assets and liabilities is as follows: March 31, December 31, 2020 2019 Operating Lease Assets $ 13,715 $ 14,110 Operating Lease Liabilities $ 14,128 $ 14,466 Weighted Average Remaining Term in years 4.77 5.01 Weighted Average discount rate used to measure outstanding lease liabilities 8.32 % 8.19 %
Supplemental Cash Flow Information Related to Operating Leases	Supplemental cash flow information related to operating leases for the three months ended March 31 is as follows: 2020 2019 Operating cash flows for operating leases $ 968 $ 331 Operating lease assets obtained in exchange for lease obligations $ 334 $ 6,940
Maturities of Operating Lease Liabilities	The maturities of the Company’s operating lease liabilities as of March 31, 2020 are as follows: 2020 $ 2,883 2021 3,518 2022 3,283 2023 3,288 2024 and thereafter 4,107 Total future minimum lease payments less 17,079 Imputed interest (2,951 ) Total operating lease liabilities $ 14,128

Marketable Securities (Tables)

Marketable Securities (Tables)	3 Months Ended
	Mar. 31, 2020
Investments Debt And Equity Securities [Abstract]
Schedule of Amortized Cost, Gross Unrealized Holding Losses and Fair Value of Available-for-Sale Debt Securities by Type of Security	The amortized cost, gross unrealized holding losses and fair value of available-for-sale debt securities by type of security as of March 31, 2020 were as follows: For the Three Months Ended March 31, 2020 Amortized Cost Gross Unrealized Holding Gains Gross Unrealized Holding Losses Fair Value U.S. Government-related obligations $ 227,419 $ 1,394 $ — $ 228,813

Other Comprehensive Income an_2

Other Comprehensive Income and Accumulated Other Comprehensive Income (Tables)	3 Months Ended
	Mar. 31, 2020
Equity [Abstract]
Summary of Changes in Accumulated Other Comprehensive Income/(Loss), by Component	The following tables summarize the changes in accumulated other comprehensive income/(loss), by component for the three months ended March 31, 2020 and 2019: Unrealized Gains (Losses) from Marketable Securities Foreign Currency Translation Adjustment Total Accumulated Other Comprehensive Income (Loss) Balances, December 31, 2019 $ — $ (154 ) $ (154 ) Net other comprehensive income (loss) 1,394 (230 ) 1,164 Balances, March 31, 2020 $ 1,394 $ (384 ) $ 1,010 Unrealized Gains (Losses) from Marketable Securities Foreign Currency Translation Adjustment Total Accumulated Other Comprehensive Income (Loss) Balances, December 31, 2018 $ — $ (123 ) $ (123 ) Net other comprehensive income (loss) — 2 2 Balances, March 31, 2019 $ — $ (121 ) $ (121 )

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2020
Fair Value, Nonrecurring [Member]
Schedule of Financial Instruments and the Related Fair Value Hierarchy of the Valuation Techniques Utilized	The following table presents the fair value of financial instruments recorded originally at amortized cost or fair value and not remeasured on a recurring basis: March 31, 2020 Balance Sheet Classification Type of Instrument Level 1 Level 2 Level 3 Total Financial Assets: Cash and cash equivalents: Money market funds $ 256,106 $ — $ — $ 256,106 Bank certificates of deposit 40,811 — — 40,811 Total Financial Assets $ 296,917 $ — $ — $ 296,917 Financial Liabilities: Convertible senior notes Convertible senior notes* $ — $ — $ 155,008 $ 155,008 Total Financial Liabilities $ — $ — $ 155,008 $ 155,008 December 31, 2019 Balance Sheet Classification Type of Instrument Level 1 Level 2 Level 3 Total Financial Assets: Cash and cash equivalents Money market funds $ 281,314 $ — $ — $ 281,314 Total Financial Assets $ 281,314 $ — $ — $ 281,314 Financial Liabilities: Convertible senior notes Convertible senior notes* $ — $ — $ 149,496 $ 149,496 Total Financial Liabilities $ — $ — $ 149,496 $ 149,496 * The convertible notes were measured and recognized at fair value on the consolidated balance sheet at inception.
Fair Value, Recurring [Member]
Schedule of Financial Instruments and the Related Fair Value Hierarchy of the Valuation Techniques Utilized	The following table presents the fair value of financial instruments recorded at fair value at inception and remeasured on a recurring basis: March 31, 2020 Balance Sheet Classification Type of Instrument Level 1 Level 2 Level 3 Total Financial Assets: Cash and cash equivalents: US government obligations $ 117,184 $ — $ — $ 117,184 Marketable Securities: US government obligations 228,813 — — 228,813 Total Financial Assets $ 345,997 $ — $ — $ 345,997 Financial Liabilities: Development derivative liability Development derivative liability $ — $ — $ 223,245 $ 223,245 Total Financial Liabilities $ — $ — $ 223,245 $ 223,245 December 31, 2019 Balance Sheet Classification Type of Instrument Level 1 Level 2 Level 3 Total Financial Liabilities: Development derivative liability Development derivative liability $ — $ — $ 134,839 $ 134,839 Total Financial Liabilities $ — $ — $ 134,839 $ 134,839

Net Loss per Share (Tables)

Net Loss per Share (Tables)	3 Months Ended
	Mar. 31, 2020
Earnings Per Share [Abstract]
Summary of Shares Outstanding that were Excluded from Calculation of Diluted Net Loss Per Share	Convertible notes and shares outstanding presented below were excluded from the calculation of diluted net loss per share, prior to the use of the treasury stock method, as their effect is anti-dilutive: For the Three Months Ended March 31, 2020 2019 Convertible notes 5,575 — Common stock options 9,434 9,191 Restricted stock units 298 — Common stock warrants — 14 Total 15,307 9,205

Nature of Organization and Op_2

Nature of Organization and Operations - Additional Information (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	Jan. 13, 2020	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Nature Of Organization And Operations [Line Items]
Proceeds from follow-on public offering after deducting underwriting discounts and commissions		$ 381,593	$ 109,927
Cash and cash equivalents		417,881	$ 288,247	$ 351,985
Marketable securities		$ 228,813
Follow-on Public Offerings [Member]
Nature Of Organization And Operations [Line Items]
Issuance of common stock in follow-on offering, shares	10,925
Price of common stock	$ 37
Proceeds from follow-on public offering after deducting underwriting discounts and commissions	$ 381,500
Underwriting discounts and commissions	22,200
Offering Cost	$ 500
Over Allotment Option [Member]
Nature Of Organization And Operations [Line Items]
Issuance of common stock in follow-on offering, shares	1,425
Price of common stock	$ 37

Basis of Presentation - Additio

Basis of Presentation - Additional Information (Detail) $ in Thousands	Mar. 31, 2020USD ($)
Accounting Policies [Abstract]
Restricted cash	$ 1,016

Accrued Expenses - Schedule of

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Accrued Liabilities Current [Abstract]
Accrued research and development	$ 30,030	$ 36,449
Accrued payroll liabilities	7,898	11,443
Other	7,002	6,891
Total	$ 44,930	$ 54,783

Development Derivative Liabil_3

Development Derivative Liability - Additional Information (Detail)	Mar. 31, 2020USD ($)	Jan. 29, 2020USD ($)	Sep. 23, 2019USD ($)	Jun. 27, 2019USD ($)	Feb. 28, 2019USD ($)Installment	Feb. 28, 2019USD ($)Installment	Jan. 31, 2020USD ($)	Mar. 31, 2020USD ($)	Mar. 31, 2019USD ($)	Dec. 31, 2019USD ($)	Jun. 07, 2019USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Minimum payment of additional funding amount								$ 20,000,000	$ 60,000,000
Loss from remeasurement of development derivative liability								68,406,000	736,000
Development derivative liability	$ 223,245,000							$ 223,245,000	60,736,000	$ 134,839,000
Level 3 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Implied cost of borrowing discount rates								13.47%
SFJ Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Proceeds from SFJ agreement				$ 20,000,000		$ 60,000,000	$ 20,000,000		60,000,000
Additional funding amount upon achievement of development milestones		$ 20,000,000	$ 20,000,000	40,000,000	$ 60,000,000	$ 60,000,000
Number of milestone payments | Installment					3	3
Development funding for minimum period of operating expense						10 months
Additional funding for development costs	50,000,000
Increase in additional funding for development costs											$ 20,000,000
Proceeds SFJ agreement upon achievement of milestone				$ 20,000,000
Number of additional annual payments | Installment						6
Minimum payment of additional funding amount						$ 120,000,000
Increase in additional funding amount under amendment											$ 20,000,000
Loss from remeasurement of development derivative liability								$ 68,400,000	$ 700,000
Proceeds from collaborators of additional milestones										60,000,000
Development derivative liability	$ 223,200,000							$ 223,200,000		$ 134,800,000
SFJ Agreement [Member] | Level 3 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Implied cost of borrowing discount rates								8.00%
SFJ Agreement [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Obligated to pay initial payment					$ 5,000,000	5,000,000
Aggregate amount of additional annual payments					226,000,000	226,000,000
SFJ Agreement [Member] | Regulatory Approval Granted by FDA and EMA [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Obligated to pay initial payment					10,000,000	10,000,000
SFJ Agreement [Member] | Regulatory Approval Granted by FDA and EMA [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate amount of additional annual payments					$ 452,000,000	$ 452,000,000

Development Derivative Liabil_4

Development Derivative Liability - Schedule of Development Derivative Liability (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Balance at fair market value, January 1,	$ 134,839
Amounts received under the SFJ agreement and SFJ amendment	20,000	$ 60,000
Loss recorded in loss from remeasurement of development derivative liability	68,406	736
Balance at fair market value, March 31,	$ 223,245	$ 60,736

Long-term Debt - Additional Inf

Long-term Debt - Additional Information (Detail) $ / shares in Units, $ in Thousands	Sep. 16, 2019USD ($)	Mar. 26, 2019USD ($)	Oct. 20, 2017USD ($)Installment$ / sharesshares	Oct. 19, 2017USD ($)$ / sharesshares	Mar. 31, 2020USD ($)d$ / shares	Mar. 31, 2019USD ($)	Sep. 30, 2019USD ($)	Dec. 31, 2019USD ($)	Sep. 11, 2019USD ($)CounterParty$ / shares
Debt Instrument [Line Items]
Amortization of debt discounts						$ 19
Premium paid for capped call transactions							$ 28,400
Loss on extinguishment of debt						$ 1,208
Term Loan Facility [Member]
Debt Instrument [Line Items]
Principal amount		$ 21,700
Final payment due amount		21,800
Debt instrument per diem interest amount		$ 100
Final payment percentage equal to original principal amount		8.00%
Final payment towards original principal amount		$ 1,600
Prepayment fee		100
Loss on extinguishment of debt		$ 1,200
Silicon Valley Bank [Member] | Term Loan Facility [Member]
Debt Instrument [Line Items]
Warrant to purchase common stock | shares			14,064,000
Exercise price per share | $ / shares			$ 5.484
Warrant term			10 years
Aggregate purchase price			$ 250
Loan and Security Agreement [Member] | Silicon Valley Bank [Member]
Debt Instrument [Line Items]
Debt instrument, due and payment description					the Company was required to make monthly interest-only payments through November 1, 2019 and was required to make 24 equal monthly payments of principal, plus accrued interest, from November 1, 2019 through October 1, 2021, when all unpaid principal and interest became due and payable
Term loan facility			$ 20,000
Loan and Security Agreement [Member] | Silicon Valley Bank [Member] | Convertible Preferred Stock [Member]
Debt Instrument [Line Items]
Debt instrument, number of equal monthly installments | Installment			24
Loan and Security Agreement [Member] | Silicon Valley Bank [Member] | Prime Rate [Member]
Debt Instrument [Line Items]
Interest bear rate			1.50%
Capped Call Transactions [Member]
Debt Instrument [Line Items]
Number of counterparties | CounterParty									2
Initial conversion price | $ / shares									$ 39.4625
Initial cap price | $ / shares									63.14
Sale price | $ / shares									$ 31.57
Convertible Senior Notes Due 2026 [Member]
Debt Instrument [Line Items]
Principal amount					$ 220,000
Net proceeds from the sale of the notes					212,900
Payment of convertible debt discounts and commissions					6,600
Payment of convertible debt offering expenses					500
Payments of transactions cost					$ 28,400
Debt instrument, stated percentage	3.50%
Debt instrument, due and payment description					The Convertible Notes are senior unsecured obligations of the Company and bear interest at a rate of 3.5% per year payable semiannually in arrears on March 15 and September 15 of each year, beginning on March 15, 2020.
Debt instrument, frequency of periodic payment					semiannually
Maturity date	Sep. 15, 2026
Redemption period, start date					Sep. 20, 2023
Threshold percentage of stock price trigger					130.00%
Threshold trading days | d					20
Threshold consecutive trading days | d					30
Redemption price, percentage					100.00%
Debt instrument effective interest rate									10.50%
Debt instrument convertible carrying amount of liability component									$ 145,100
Debt discount									74,900
Debt issuance costs									7,100
Debt interest expense					$ 3,900
Amortization of debt discounts					1,900
Accrued semi annual coupon payable					1,900
Amortization of debt issuance costs					100
Debt issuance costs gross					700
Long-term debt					$ 144,600			$ 142,600
Convertible Senior Notes Due 2026 [Member] | Liability Component [Member]
Debt Instrument [Line Items]
Debt issuance costs									4,700
Convertible Senior Notes Due 2026 [Member] | Equity Component [Member]
Debt Instrument [Line Items]
Debt issuance costs									$ 2,400
Convertible Senior Notes Due 2026 [Member] | Common Stock [Member]
Debt Instrument [Line Items]
Terms of conversion					The Convertible Notes are convertible into shares of the Company’s common stock at an initial conversion rate of 25.3405 shares per $1,000 principal amount of Convertible Notes (equivalent to an initial conversion price of approximately $39.46 per share of common stock).
Conversion ratio					0.0253405
Conversion price | $ / shares					$ 39.46
Convertible Senior Notes Due 2026 [Member] | Private Offering [Member]
Debt Instrument [Line Items]
Principal amount	$ 220,000
Unsecured Promissory Note [Member] | Golda Darty Partners S A [Member]
Debt Instrument [Line Items]
Principal amount	7,000			$ 7,000
Debt instrument, due and payment description					The promissory note accrued interest at a rate per annum of 8.0%, and was due and payable quarterly in arrears on the 19th day of each April, July, October and January.
Maturity date				Oct. 19, 2022
Long-term debt				$ 7,000
Debt instrument per diem interest amount	100
Loss on extinguishment of debt				$ 300
Warrant to purchase common stock | shares				93,764,000
Exercise price per share | $ / shares				$ 5.484
Accrued interest rate				8.00%
Repayment of related party due amount including diem interest	$ 7,100
Fair value of warrant				$ 400

Leases - Additional Information

Leases - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Jan. 01, 2019
Lessee Lease Description [Line Items]
Operating lease asset	$ 13,715		$ 14,110
Operating lease liability	$ 14,128		$ 14,466
Operating lease description	The Company enters into lease agreements with terms generally ranging from 2-7 years. Some of the Company’s lease agreements include Company options to extend the lease on a month to month basis or for set periods for up to five years. Many leases also include options to terminate the leases within one year or per other contractual terms. Renewal and termination options were generally not included in the lease term for the Company’s existing operating leases.
Operating lease, existence of option to extend	true
Operating lease maximum term of options to terminate lease	1 year
Operating lease expense	$ 1,000	$ 400
Minimum [Member]
Lessee Lease Description [Line Items]
Operating lease, term of contract	2 years
Maximum [Member]
Lessee Lease Description [Line Items]
Operating lease, term of contract	7 years
Operating lease options to extend lease	5 years
ASU 2016-02 [Member]
Lessee Lease Description [Line Items]
Operating lease asset				$ 5,500
Operating lease liability				5,500
Additional operating lease asset				9,800
Additional operating lease liability				$ 9,800

Leases - Schedule of Additional

Leases - Schedule of Additional Information Related to Operating Lease Assets and Liabilities (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating Lease Assets	$ 13,715	$ 14,110
Operating Lease Liabilities	$ 14,128	$ 14,466
Weighted Average Remaining Term in years	4 years 9 months 7 days	5 years 3 days
Weighted Average discount rate used to measure outstanding lease liabilities	8.32%	8.19%

Leases - Supplemental Cash Flow

Leases - Supplemental Cash Flow Information Related to Operating Leases (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Leases [Abstract]
Operating cash flows for operating leases	$ 968	$ 331
Operating lease assets obtained in exchange for lease obligations	$ 334	$ 6,940

Leases - Maturities of Operatin

Leases - Maturities of Operating Lease Liabilities (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Leases [Abstract]
2020	$ 2,883
2021	3,518
2022	3,283
2023	3,288
2024 and thereafter	4,107
Total future minimum lease payments less	17,079
Imputed interest	(2,951)
Operating Lease Liabilities	$ 14,128	$ 14,466

Marketable Securities - Schedul

Marketable Securities - Schedule of Amortized Cost, Gross Unrealized Holding Losses and Fair Value of Available-for-Sale Debt Securities by Type of Security (Detail) - USD ($)	Mar. 31, 2020	Dec. 31, 2019
Available for sale securities:
Fair Value		$ 0
U.S. Government-Related Obligations [Member]
Available for sale securities:
Amortized Cost	$ 227,419,000
Gross Unrealized Holding Gains	1,394,000
Fair Value	$ 228,813,000

Marketable Securities - Additio

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

Other Comprehensive Income an_3

Other Comprehensive Income and Accumulated Other Comprehensive Income - Summary of Changes in Accumulated Other Comprehensive Income/(Loss), by Component (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning balance	$ 34,229	$ 160,972
Net other comprehensive income (loss)	1,164	2
Ending balance	258,997	224,755
Unrealized Gains (Losses) from Marketable Securities [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Net other comprehensive income (loss)	1,394
Ending balance	1,394
Foreign Currency Translation Adjustment [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning balance	(154)	(123)
Net other comprehensive income (loss)	(230)	2
Ending balance	(384)	(121)
Total Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning balance	(154)	(123)
Ending balance	$ 1,010	$ (121)

Fair Value Measurements - Sched

Fair Value Measurements - Schedule of Financial Instruments and the Related Fair Value Hierarchy of the Valuation Techniques Utilized (Detail) - Fair Value, Nonrecurring [Member] - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Assets	$ 296,917	$ 281,314
Total Financial Liabilities	155,008	149,496
Money Market Funds [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Assets	256,106	281,314
Bank Certificates of Deposit [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Assets	40,811
Convertible Senior Notes [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Liabilities	155,008	149,496
Level 1 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Assets	296,917	281,314
Level 1 [Member] | Money Market Funds [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Assets	256,106	281,314
Level 1 [Member] | Bank Certificates of Deposit [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Assets	40,811
Level 3 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Liabilities	155,008	149,496
Level 3 [Member] | Convertible Senior Notes [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Liabilities	$ 155,008	$ 149,496

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) - Level 3 [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Percentage of cost of borrowing	13.47%
SFJ Agreement [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Percentage of cost of borrowing	8.00%
Convertible Senior Notes [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Convertible notes	$ 155	$ 149.5
Convertible notes, interest rate	3.50%	3.50%

Fair Value Measurements - Sch_2

Fair Value Measurements - Schedule of Financial Instruments and the Related Fair Value Hierarchy of the Valuation Techniques Utilized (Details) - Fair Value, Recurring [Member] - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Assets	$ 345,997
Total Financial Liabilities	223,245	$ 134,839
Cash and Cash Equivalents [Member] | US Government Obligations [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Assets	117,184
Marketable Securities [Member] | US Government Obligations [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Assets	228,813
Development Derivative Liability [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Liabilities	223,245	134,839
Level 1 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Assets	345,997
Level 1 [Member] | Cash and Cash Equivalents [Member] | US Government Obligations [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Assets	117,184
Level 1 [Member] | Marketable Securities [Member] | US Government Obligations [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Assets	228,813
Level 3 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Liabilities	223,245	134,839
Level 3 [Member] | Development Derivative Liability [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Liabilities	$ 223,245	$ 134,839

Refundable Research and Devel_2

Refundable Research and Development Credit and Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Income Tax Expense Benefit Continuing Operations [Abstract]
Current income tax expense or benefits	$ 0	$ 0
Deferred income tax expense or benefits	$ 0	$ 0

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2020USD ($)ft²
Commitments And Contingencies [Line Items]
Base rent payable	$ 17,079,000
Estimated termination costs and other fees	2,800,000
Non-cancellable purchase commitments	$ 22,600,000
Minimum [Member]
Commitments And Contingencies [Line Items]
Operating lease, term of contract	2 years
Payment of royalty fees	3.00%
Maximum [Member]
Commitments And Contingencies [Line Items]
Operating lease, term of contract	7 years
Future milestone payments	$ 15,000,000
Payment of royalty fees	6.00%
Watertown, Massachusetts [Member]
Commitments And Contingencies [Line Items]
Operating lease, term of contract	85 months
Area of operating lease | ft²	9,704
Initial monthly lease payments per month	$ 60,000
Base rent payable	$ 5,400,000
Lease commencement date	Jun. 2, 2020

Net Loss per Share - Summary of

Net Loss per Share - Summary of Shares Outstanding that were Excluded from Calculation of Diluted Net Loss Per Share (Detail) - shares shares in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount	15,307	9,205
Convertible Notes [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount	5,575
Common Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount	9,434	9,191
Restricted Stock Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount	298
Common Stock Warrants [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount		14

Related Party Transaction - Add

Related Party Transaction - Additional Information (Detail) - Revon [Member] - License Agreement and Services Agreement [Member] $ in Millions	May 01, 2018USD ($)
Related Party Transaction [Line Items]
Annual license fee	$ 0.2
Related party transaction, initial term	2 years
Exclusive license option, thereafter	$ 0.4
Development service costs	$ 0.3