Quarterly Financial Data (unaudited) - Summary of Unaudited Quarterly Financial Data (Details) - USD ($)	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
Quarterly Financial Data [Abstract]
Revenues					$ 12,400,000	$ 9,188,000	$ 6,639,000	$ 5,331,000	$ 0	$ 33,558,000	$ 11,442,000
Total operating expenses	$ 61,411,000	$ 145,118,000	$ 41,564,000	$ 39,741,000	30,281,000	26,867,000	26,130,000	23,931,000	287,834,000	107,209,000	70,109,000
Net loss	$ (58,767,000)	$ (141,802,000)	$ (38,174,000)	$ (37,470,000)	$ (15,676,000)	$ (15,789,000)	$ (18,413,000)	$ (17,820,000)	$ (276,213,000)	$ (67,698,000)	$ (57,010,000)
Net loss per common share, basic and diluted	$ (1.27)	$ (3.07)	$ (0.83)	$ (0.93)	$ (0.39)	$ (0.39)	$ (0.49)	$ (0.50)	$ (6.17)	$ (1.76)	$ (1.74)
Weighted average number of shares, basic and diluted	46,278,409	46,133,068	46,065,901	40,506,313	40,259,575	40,116,644	37,440,024	35,827,235	44,765,496	38,386,906	32,832,514