Selected Unaudited Quarterly Financial Data - Summary of Unaudited Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 1,562	$ 4,291	$ 1,250	$ 1,250	$ 1,250	$ 163			$ 8,353	$ 1,413
Total operating expenses	15,046	16,436	17,149	14,581	14,594	23,297	$ 9,664	$ 7,300	63,212	54,854	$ 18,182
Net loss	$ (12,295)	$ (10,812)	$ (13,533)	$ (12,954)	$ (13,210)	$ (23,122)	$ (9,550)	$ (7,200)	$ (49,594)	$ (53,082)	$ (17,970)
Net loss per share — basic and diluted	$ (0.28)	$ (0.25)	$ (0.32)	$ (1.37)	$ (5.98)	$ (11.86)	$ (5.64)	$ (4.96)	$ (1.43)	$ (29.03)	$ (20.80)
Weighted-average number of shares used to compute basic and diluted net loss per share	43,163,412	42,838,098	42,533,641	9,488,352	2,208,065	1,949,258	1,693,150	1,452,215	34,618,237	1,828,262	863,983