Selected Quarterly Financial Data (Unaudited) - Summary of Quarterly Financial Information (Detail) - 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	$ 1,960	$ 3,570	$ 873	$ 465	$ 273		$ 90,000		$ 6,868	$ 90,273
Total operating costs and expenses	176,598	190,747	177,330	170,317	164,500	$ 128,745	112,147	$ 78,119	714,992	483,511	$ 273,155
Loss from operations	(174,638)	(187,177)	(176,457)	(169,852)	(164,227)	(128,745)	(22,147)	(78,119)	(708,124)	(393,238)	(273,155)
Net loss	$ (168,653)	$ (179,958)	$ (168,221)	$ (163,406)	$ (158,388)	$ (122,918)	$ (16,978)	$ (74,598)	$ (680,238)	$ (372,882)	$ (270,120)
Net loss per share—basic and diluted	$ (3.25)	$ (3.48)	$ (3.28)	$ (3.37)	$ (3.38)	$ (2.63)	$ (0.36)	$ (1.68)	$ (13.38)	$ (8.08)	$ (7.09)