Condensed Statement of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total		Common Stock	Additional Paid-in Capital	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]
Stock-based compensation	7,867			7,867
Employee stock purchase plan (in shares)			44,649
Employee stock purchase plan	684			684
Net loss	(31,586)				(31,586)
Net unrealized gain on available-for-sale investments	1,099					1,099
Ending balance (in shares) at Mar. 31, 2019			121,527,320
Ending balance at Mar. 31, 2019	$ 681,228		$ 121	922,816	(243,114)	1,405
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)			339,775
Issuance of common stock upon exercise of stock options and vesting of RSU's	303		$ 0	303
Vesting of early exercised common stock	710			710
Stock-based compensation	14,215			14,215
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
Issuance of common stock from public offering, net of commissions and offering costs of $0.3 million	14,845		$ 1	14,844
Net loss	(54,480)				(54,480)
Net unrealized gain on available-for-sale investments	$ 465					465
Ending balance (in shares) at Mar. 31, 2020	125,262,537		125,262,537
Ending balance at Mar. 31, 2020	$ 606,519		$ 125	$ 1,055,386	$ (450,602)	$ 1,610

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