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, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 9,487	$ 64,530	$ 1,750	$ 1,750	$ 9,750	$ 1,750	$ 1,750	$ 1,750
Total operating expenses	61,499	62,978	47,125	30,150	27,277	24,999	30,910	20,523	$ 201,752	$ 103,709	$ 63,212
Net income (loss)	$ (51,853)	$ 1,822	$ (45,074)	$ (27,753)	$ (16,598)	$ (22,352)	$ (28,090)	$ (17,670)	$ (122,858)	$ (84,710)	$ (49,594)
Net income (loss) per share — basic	$ (0.82)	$ 0.03	$ (0.93)	$ (0.63)
Net income (loss) per share — diluted	$ (0.82)	$ 0.03	$ (0.93)	$ (0.63)
Weighted average number of shares, basic	63,527,932	62,599,193	48,556,843	44,282,607
Weighted average number of shares, diluted	63,527,932	65,145,707	48,556,843	44,282,607
Net loss per share — basic and diluted					$ (0.38)	$ (0.51)	$ (0.64)	$ (0.41)	$ (2.24)	$ (1.93)	$ (1.43)
Weighted average number of shares, basic and diluted					44,056,407	43,939,281	43,797,718	43,508,592	54,787,118	43,825,991	34,618,237