Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Common stock, shares outstanding beginning balance (in shares) at Dec. 31, 2018		94,662,435
Beginning balance at Dec. 31, 2018	$ 546,845	$ 1,273	$ 774,158	$ (649)	$ (227,937)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuances under equity incentive plans (in shares)		199,503
Issuances under equity incentive plans	908	$ 2	906
Vesting of early exercised common stock (in shares)		46,874
Vesting of early exercised common stock	32	$ 1	31
Vesting of restricted stock awards and units (in shares)		348,893
Vesting of restricted stock awards	0	$ 3	(3)
Stock-based compensation	6,874		6,874
Net loss	(38,992)				(38,992)
Other comprehensive lncome	981			981
Common stock, shares outstanding ending balance (in shares) at Mar. 31, 2019		95,257,705
Ending balance at Mar. 31, 2019	$ 516,648	$ 1,279	781,966	332	(266,929)
Common stock, shares outstanding beginning balance (in shares) at Dec. 31, 2019	96,189,935	96,189,935
Beginning balance at Dec. 31, 2019	$ 394,890	$ 1,288	818,803	350	(425,551)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock in follow-on offering, net of issuance costs of $632 (in shares)		9,000,000
Issuance of common stock in follow-on offering, net of issuance costs of $632	193,948	$ 90	193,858
Issuances under equity incentive plans (in shares)		259,569
Issuances under equity incentive plans	1,966	$ 3	1,963
Vesting of restricted stock awards and units (in shares)		91,839
Vesting of restricted stock awards	0	$ 1	(1)
Stock-based compensation	10,785		10,785
Net loss	(56,763)				(56,763)
Other comprehensive lncome	$ 485			485
Common stock, shares outstanding ending balance (in shares) at Mar. 31, 2020	105,541,343	105,541,343
Ending balance at Mar. 31, 2020	$ 545,311	$ 1,382	$ 1,025,408	$ 835	$ (482,314)