Condensed Consolidated Statements of Stockholders’ Equity - USD ($) $ in Thousands	Total		ATM offering	Public offering	Common Stock	Common Stock ATM offering	Common Stock Public offering	Additional Paid-in Capital	Additional Paid-in Capital ATM offering	Additional Paid-in Capital Public offering	Accumulated Deficit	Accumulated Other Comprehensive Income
Beginning balance (in shares) at Dec. 31, 2018					121,482,671
Beginning balance at Dec. 31, 2018	$ 703,164				$ 121			$ 914,265			$ (211,528)	$ 306
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options and vesting of RSU's (in shares)					304,826
Issuance of common stock upon exercise of stock options and vesting of RSUs	704				$ 1			703
Vesting of early exercised common stock	3,880							3,880
Stock-based compensation	32,189							32,189
Employee stock purchase plan (in shares)					107,982
Employee stock purchase plan	1,783							1,783
Net loss	(123,564)										(123,564)
Net unrealized gain (loss) on available-for-sale investments	1,560											1,560
Ending balance (in shares) at Sep. 30, 2019					121,895,479
Ending balance at Sep. 30, 2019	619,716				$ 122			952,820			(335,092)	1,866
Beginning balance (in shares) at Jun. 30, 2019					121,631,278
Beginning balance at Jun. 30, 2019	655,635				$ 122			937,709			(284,357)	2,161
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options and vesting of RSU's (in shares)					200,868
Issuance of common stock upon exercise of stock options and vesting of RSUs	467							467
Vesting of early exercised common stock	710							710
Stock-based compensation	12,835							12,835
Employee stock purchase plan (in shares)					63,333
Employee stock purchase plan	1,099							1,099
Net loss	(50,735)										(50,735)
Net unrealized gain (loss) on available-for-sale investments	(295)											(295)
Ending balance (in shares) at Sep. 30, 2019					121,895,479
Ending balance at Sep. 30, 2019	$ 619,716				$ 122			952,820			(335,092)	1,866
Beginning balance (in shares) at Dec. 31, 2019	124,267,358				124,267,358
Beginning balance at Dec. 31, 2019	$ 629,023	[1]			$ 124			1,023,876			(396,122)	1,145
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options and vesting of RSU's (in shares)					1,285,053
Issuance of common stock upon exercise of stock options and vesting of RSUs	7,227				$ 2			7,225
Vesting of early exercised common stock	2,130							2,130
Stock-based compensation	48,808							48,808
Employee stock purchase plan (in shares)					175,142
Employee stock purchase plan	2,843							2,843
Issuance of common stock from public offering (in shares)						570,839	13,457,447
Issuance of common stock from public offering			$ 14,845	$ 595,698		$ 1	$ 13		$ 14,844	$ 595,685
Net loss	(181,651)										(181,651)
Net unrealized gain (loss) on available-for-sale investments	$ (192)											(192)
Ending balance (in shares) at Sep. 30, 2020	139,755,839				139,755,839
Ending balance at Sep. 30, 2020	$ 1,118,731				$ 140			1,695,411			(577,773)	953
Beginning balance (in shares) at Jun. 30, 2020					139,233,450
Beginning balance at Jun. 30, 2020	1,163,419				$ 139			1,673,026			(511,576)	1,830
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options and vesting of RSU's (in shares)					431,812
Issuance of common stock upon exercise of stock options and vesting of RSUs	2,447				$ 1			2,446
Vesting of early exercised common stock	710							710
Stock-based compensation	17,823							17,823
Employee stock purchase plan (in shares)					90,577
Employee stock purchase plan	1,406							1,406
Net loss	(66,197)										(66,197)
Net unrealized gain (loss) on available-for-sale investments	$ (877)											(877)
Ending balance (in shares) at Sep. 30, 2020	139,755,839				139,755,839
Ending balance at Sep. 30, 2020	$ 1,118,731				$ 140			$ 1,695,411			$ (577,773)	$ 953

[1] The balance sheet as of December 31, 2019 is derived from the audited financial statements as of that date.