Condensed Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Unaudited) - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Series B Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance at Dec. 31, 2018	$ (179,177)			$ 1	$ 14,672	$ (14)	$ (193,836)
Beginning balance (in shares) at Dec. 31, 2018		69,910,520
Beginning balance at Dec. 31, 2018		$ 309,137
Beginning balance (in shares) at Dec. 31, 2018				8,858,799
Issuance of convertible preferred stock, net of issuance cost			$ 327,475
Issuance of convertible preferred stock, net of issuance cost (in Shares)			18,202,213
Issuance of common stock in connection with a license agreement	617				617
Issuance of common stock in connection with a license agreement (in shares)				38,888
Repayment of promissory notes, net of unvested common stock	1,355				1,355
Repayment of promissory notes, net of unvested shares (in shares)				1,390,925
Vesting of restricted common stock	119				119
Vesting of restricted common stock (in shares)				733,041
Exercise of stock options	1,066				1,066
Exercise of stock option (in shares)				706,579
Stock-based compensation	6,040				6,040
Other comprehensive income (loss)	95					95
Net loss	(110,912)						(110,912)
Ending balance at Sep. 30, 2019	(280,797)			$ 1	23,869	81	(304,748)
Ending balance (in shares) at Sep. 30, 2019		88,112,733
Ending balance at Sep. 30, 2019		$ 636,612
Ending balance (in shares) at Sep. 30, 2019				11,728,232
Beginning balance at Jun. 30, 2019	(236,967)			$ 1	19,226	240	(256,434)
Beginning balance (in shares) at Jun. 30, 2019		88,112,733
Beginning balance at Jun. 30, 2019		$ 636,612
Beginning balance (in shares) at Jun. 30, 2019				9,722,838
Issuance of common stock in connection with a license agreement	617				617
Issuance of common stock in connection with a license agreement (in shares)				38,888
Repayment of promissory notes, net of unvested common stock	1,355				1,355
Repayment of promissory notes, net of unvested shares (in shares)				1,390,925
Vesting of restricted common stock	119				119
Vesting of restricted common stock (in shares)				337,075
Exercise of stock options	364				364
Exercise of stock option (in shares)				238,506
Stock-based compensation	2,188				2,188
Other comprehensive income (loss)	(159)					(159)
Net loss	(48,314)						(48,314)
Ending balance at Sep. 30, 2019	(280,797)			$ 1	23,869	81	(304,748)
Ending balance (in shares) at Sep. 30, 2019		88,112,733
Ending balance at Sep. 30, 2019		$ 636,612
Ending balance (in shares) at Sep. 30, 2019				11,728,232
Beginning balance at Dec. 31, 2019	$ 423,942			$ 11	793,051	(601)	(368,519)
Beginning balance (in shares) at Dec. 31, 2019	107,648,925			107,648,925
Reclassification of derivative liability to additional paid-in capital	$ 29,245				29,245
Issuance of common stock in connection with the achievement of a milestone (in shares)				1,111,111
Issuance of common stock in connection with a collaboration agreement	206,699			$ 1	206,698
Issuance of common stock in connection with a collaboration agreement (in shares)				6,626,027
Issuance of common stock for cashless exercise of warrant (in shares)				211,774
Issuance of common stock in connection with a follow-on offering	323,214			$ 1	323,213
Issuance of common stock in connection with a follow-on offering (in shares)				8,214,285
Vesting of restricted common stock	1,316				1,316
Vesting of restricted common stock (in shares)				1,791,880
Exercise of stock options	3,540				3,540
Exercise of stock option (in shares)				1,387,629
Stock-based compensation	17,299				17,299
Other comprehensive income (loss)	116					116
Net loss	(193,016)						(193,016)
Ending balance at Sep. 30, 2020	$ 812,355			$ 13	1,374,362	(485)	(561,535)
Ending balance (in shares) at Sep. 30, 2020	126,991,631			126,991,631
Beginning balance at Jun. 30, 2020	$ 563,779			$ 12	1,040,988	(295)	(476,926)
Beginning balance (in shares) at Jun. 30, 2020				117,727,086
Issuance of common stock in connection with a follow-on offering	323,214			$ 1	323,213
Issuance of common stock in connection with a follow-on offering (in shares)				8,214,285
Vesting of restricted common stock	427				427
Vesting of restricted common stock (in shares)				561,255
Exercise of stock options	1,152				1,152
Exercise of stock option (in shares)				489,005
Stock-based compensation	8,582				8,582
Other comprehensive income (loss)	(190)					(190)
Net loss	(84,609)						(84,609)
Ending balance at Sep. 30, 2020	$ 812,355			$ 13	$ 1,374,362	$ (485)	$ (561,535)
Ending balance (in shares) at Sep. 30, 2020	126,991,631			126,991,631