Selected Quarterly Financial Data (Unaudited) (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, net	$ 24,683	$ 20,081	$ 18,033	$ 20,341	$ 24,657	$ 18,544	$ 15,835	$ 10,852	$ 83,138	$ 69,888	$ 24,181
Total costs and expenses	60,276	53,685	60,586	64,281	74,595	61,871	61,910	59,004	238,828	257,380	233,923
Net loss	$ (46,137)	$ (39,648)	$ (48,187)	$ (49,129)	$ (55,064)	$ (49,402)	$ (47,164)	$ (47,951)	$ (183,101)	$ (199,581)	$ (232,569)
Net loss per common share—basic and diluted (in dollars per share)	$ (1.00)	$ (0.86)	$ (1.05)	$ (1.07)	$ (1.21)	$ (1.09)	$ (1.04)	$ (1.06)	$ (3.99)	$ (4.39)	$ (5.58)