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)		547,124
Issuances under equity incentive plans	3,477	$ 5	3,472
Vesting of early exercised common stock (in shares)		93,752
Vesting of early exercised common stock	64	$ 2	62
Vesting of restricted stock units (2020) and restricted stock awards and units (2019) (in shares)		353,585
Vesting of restricted stock units (2020) and restricted stock awards and units (2019)	0	$ 3	(3)
Stock-based compensation	20,588		20,588
Net loss	(97,329)				(97,329)
Other comprehensive income (loss)	1,528			1,528
Common stock, shares outstanding ending balance (in shares) at Jun. 30, 2019		95,656,896
Ending balance at Jun. 30, 2019	475,173	$ 1,283	798,277	879	(325,266)
Common stock, shares outstanding beginning balance (in shares) at Mar. 31, 2019		95,257,705
Beginning balance at Mar. 31, 2019	516,648	$ 1,279	781,966	332	(266,929)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuances under equity incentive plans (in shares)		347,621
Issuances under equity incentive plans	2,569	$ 3	2,566
Vesting of early exercised common stock (in shares)		46,878
Vesting of early exercised common stock	32	$ 1	31
Vesting of restricted stock units (2020) and restricted stock awards and units (2019) (in shares)		4,692
Vesting of restricted stock units (2020) and restricted stock awards and units (2019)	0
Stock-based compensation	13,714		13,714
Net loss	(58,337)				(58,337)
Other comprehensive income (loss)	547			547
Common stock, shares outstanding ending balance (in shares) at Jun. 30, 2019		95,656,896
Ending balance at Jun. 30, 2019	$ 475,173	$ 1,283	798,277	879	(325,266)
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)		589,516
Issuances under equity incentive plans	5,380	$ 6	5,374
Vesting of restricted stock units (2020) and restricted stock awards and units (2019) (in shares)		118,421
Vesting of restricted stock units (2020) and restricted stock awards and units (2019)	0	$ 1	(1)
Stock-based compensation	23,269		23,269
Net loss	(115,521)				(115,521)
Other comprehensive income (loss)	$ 201			201
Common stock, shares outstanding ending balance (in shares) at Jun. 30, 2020	105,897,872	105,897,872
Ending balance at Jun. 30, 2020	$ 502,167	$ 1,385	1,041,303	551	(541,072)
Common stock, shares outstanding beginning balance (in shares) at Mar. 31, 2020		105,541,343
Beginning balance at Mar. 31, 2020	545,311	$ 1,382	1,025,408	835	(482,314)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuances under equity incentive plans (in shares)		329,947
Issuances under equity incentive plans	3,414	$ 3	3,411
Vesting of restricted stock units (2020) and restricted stock awards and units (2019) (in shares)		26,582
Vesting of restricted stock units (2020) and restricted stock awards and units (2019)	0
Stock-based compensation	12,484		12,484
Net loss	(58,758)				(58,758)
Other comprehensive income (loss)	$ (284)			(284)
Common stock, shares outstanding ending balance (in shares) at Jun. 30, 2020	105,897,872	105,897,872
Ending balance at Jun. 30, 2020	$ 502,167	$ 1,385	$ 1,041,303	$ 551	$ (541,072)