Quarterly Financial Information (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
Schedule Of Quarterly Financial Information [Line Items]
Revenue	$ 6,127	$ 7,735	$ 10,773	$ 9,653	$ 10,412	$ 8,352	$ 6,732	$ 5,680	$ 34,288	$ 31,176	$ 15,353
Net loss	$ (49,334)	$ (44,570)	$ (44,492)	$ (46,352)	$ (50,843)	$ (44,607)	$ (44,465)	$ (43,854)	$ (184,748)	$ (183,769)	$ (279,966)
Net loss per share	$ (0.13)	$ (0.12)	$ (0.12)	$ (0.14)	$ (0.16)	$ (0.15)	$ (0.16)	$ (0.16)	$ (0.50)	$ (0.63)	$ (1.03)