Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Total operating expenses	$ 17,989	$ 19,254	$ 22,315	$ 21,580	$ 23,774	$ 19,053	$ 19,117	$ 16,757	$ 81,138	$ 78,701
Total other income (expense), net	149	274	383	556	650	662	218	237	1,362	1,767
Net loss					(23,124)	(18,391)	(18,899)	(16,520)	(79,776)	(76,934)
Accretion of redeemable convertible preferred stock to redemption value					0	0	(3,185)	(3,186)	0	(6,371)
Net loss attributable to common stockholders	$ (17,840)	$ (18,980)	$ (21,932)	$ (21,024)	$ (23,124)	$ (18,391)	$ (22,084)	$ (19,706)	$ (79,776)	$ (83,305)
Net loss per share attributable to common stockholders, basic and diluted (in dollars per share)	$ (0.63)	$ (0.68)	$ (0.79)	$ (0.76)	$ (0.84)	$ (0.67)	$ (7.84)	$ (9.47)	$ (2.86)	$ (5.54)