Selected Quarterly Financial Information (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	Dec. 31, 2017
Selected Quarterly Financial Information (Unaudited)
Revenue	$ 3,618	$ 2,038	$ 4,051	$ 2,235	$ 5,080	$ 1,900	$ 649		$ 11,942	$ 7,629
Operating expenses	(16,843)	(15,614)	(17,680)	(12,890)	(7,284)	(7,452)	(7,624)	$ (6,449)
Net loss	$ (10,244)	$ (11,385)	$ (12,079)	$ (9,329)	$ (1,892)	$ (3,953)	$ (5,819)	$ (4,573)	$ (43,037)	$ (16,237)	$ (12,723)
Net loss per share—basic and diluted	$ (0.40)	$ (0.45)	$ (0.63)	$ (9.27)	$ (2.05)	$ (4.34)	$ (6.38)	$ (5.02)	$ (2.41)	$ (17.76)	$ (13.95)