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)					103,958
Issuance of common stock upon exercise of stock options and vesting of RSUs	237				$ 1			236
Vesting of early exercised common stock	3,170							3,170
Stock-based compensation	19,354							19,354
Employee stock purchase plan (in shares)					44,649
Employee stock purchase plan	684							684
Net loss	(72,829)										(72,829)
Net unrealized gain on available-for-sale investments	1,855											1,855
Ending balance (in shares) at Jun. 30, 2019					121,631,278
Ending balance at Jun. 30, 2019	655,635				$ 122			937,709			(284,357)	2,161
Beginning balance (in shares) at Mar. 31, 2019					121,527,320
Beginning balance at Mar. 31, 2019	681,228				$ 121			922,816			(243,114)	1,405
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options and vesting of RSU's (in shares)					103,958
Issuance of common stock upon exercise of stock options and vesting of RSUs	237				$ 1			236
Vesting of early exercised common stock	3,170							3,170
Stock-based compensation	11,487							11,487
Net loss	(41,243)										(41,243)
Net unrealized gain on available-for-sale investments	756											756
Ending balance (in shares) at Jun. 30, 2019					121,631,278
Ending balance at Jun. 30, 2019	$ 655,635				$ 122			937,709			(284,357)	2,161
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)					853,241
Issuance of common stock upon exercise of stock options and vesting of RSUs	4,780				$ 1			4,779
Vesting of early exercised common stock	1,419							1,419
Stock-based compensation	30,985							30,985
Employee stock purchase plan (in shares)					84,565
Employee stock purchase plan	1,438							1,438
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	(115,454)										(115,454)
Net unrealized gain on available-for-sale investments	$ 685											685
Ending balance (in shares) at Jun. 30, 2020	139,233,450				139,233,450
Ending balance at Jun. 30, 2020	$ 1,163,419				$ 139			1,673,026			(511,576)	1,830
Beginning balance (in shares) at Mar. 31, 2020					125,262,537
Beginning balance at Mar. 31, 2020	606,519				$ 125			1,055,386			(450,602)	1,610
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options and vesting of RSU's (in shares)					513,466
Issuance of common stock upon exercise of stock options and vesting of RSUs	4,476				$ 1			4,475
Vesting of early exercised common stock	710							710
Stock-based compensation	16,770							16,770
Issuance of common stock from public offering (in shares)					13,457,447
Issuance of common stock from public offering	595,698				$ 13			595,685
Net loss	(60,974)										(60,974)
Net unrealized gain on available-for-sale investments	$ 220											220
Ending balance (in shares) at Jun. 30, 2020	139,233,450				139,233,450
Ending balance at Jun. 30, 2020	$ 1,163,419				$ 139			$ 1,673,026			$ (511,576)	$ 1,830

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