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, 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 revenues	$ 9,750	$ 1,750	$ 1,750	$ 1,750	$ 1,562	$ 4,291	$ 1,250	$ 1,250	$ 15,000	$ 8,353
Total operating expenses	27,277	24,999	30,910	20,523	15,046	16,436	17,149	14,581	103,709	63,212
Net loss	$ (16,598)	$ (22,352)	$ (28,090)	$ (17,670)	$ (12,295)	$ (10,812)	$ (13,533)	$ (12,954)	$ (84,710)	$ (49,594)
Net loss per share — basic and diluted	$ (0.38)	$ (0.51)	$ (0.64)	$ (0.41)	$ (0.28)	$ (0.25)	$ (0.32)	$ (1.37)	$ (1.93)	$ (1.43)
Weighted-average number of shares used to compute basic and diluted net loss per share	44,056,407	43,939,281	43,797,718	43,508,592	43,163,412	42,838,098	42,533,641	9,488,352	43,825,991	34,618,237