Selected Quarterly Financial Data (Unaudited) - Schedule of Quarterly Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Total operating expenses	$ 64,143	$ 78,069	$ 67,406	$ 65,269	$ 71,500	$ 69,120	$ 64,123	$ 60,083	$ 274,887	$ 264,826	$ 141,063
Net loss	$ (64,913)	$ (85,604)	$ (70,568)	$ (67,904)	$ (75,014)	$ (70,633)	$ (65,770)	$ (62,134)	$ (288,989)	$ (273,551)	$ (143,255)
Net loss per common share — basic and diluted (in USD per share)	$ (0.73)	$ (0.96)	$ (0.79)	$ (0.89)	$ (1.07)	$ (1.04)	$ (0.99)	$ (0.98)	$ (3.37)	$ (4.09)	$ (2.41)
Weighted average common shares outstanding — basic and diluted (in shares)	89,130,806	88,893,579	88,798,398	76,468,347	70,076,475	67,616,419	66,666,876	63,310,177	85,839,303	66,910,060	59,520,747