Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	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 Information Disclosure [Abstract]
Total revenue	$ 14,055	$ 17,046	$ 13,083	$ 16,025	$ 35,421	$ 41,757	$ 28,851	$ 29,039	$ 60,209	$ 135,068	$ 205,825
Total operating expenses	144,611	146,861	156,792	157,665	188,452	127,575	125,866	106,441	605,929	548,334	475,181
Loss from operations	$ (130,556)	$ (129,815)	$ (143,709)	$ (141,640)	$ (153,031)	$ (85,818)	$ (97,015)	$ (77,402)	$ (545,720)	$ (413,266)	$ (269,356)
Net loss per share attributable to common stockholders, basic and diluted (usd per share)	$ (0.37)	$ (0.37)	$ (0.41)	$ (0.40)	$ (1.14)	$ (1.32)	$ (1.43)	$ (1.16)	$ (1.55)	$ (4.95)	$ (4.18)
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New accounting pronouncement or change in accounting principle, effect of change on operating results		$ 1,000	$ 200	$ 200