Selected Quarterly Financial Data (Unaudited) (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, net	$ 14,456	$ 7,302	$ 2,423	$ 0					$ 24,181	$ 0	$ 0
Total costs and expenses	66,381	68,640	58,107	40,795	$ 60,314	$ 32,182	$ 27,768	$ 25,429	233,923	145,693	96,872
Net loss	$ (51,458)	$ (85,388)	$ (55,024)	$ (40,699)	$ (58,513)	$ (32,372)	$ (28,433)	$ (25,787)	$ (232,569)	$ (145,105)	$ (99,059)
Net loss per common share—basic and diluted (in dollars per share)	$ (1.14)	$ (1.96)	$ (1.40)	$ (1.05)	$ (1.60)	$ (0.89)	$ (0.82)	$ (0.76)	$ (5.58)	$ (4.11)	$ (3.40)