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, 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 revenue	$ 273		$ 90,000						$ 90,273
Total operating expenses	164,500	$ 128,745	112,147	$ 78,119	$ 70,448	$ 74,373	$ 70,854	$ 57,480	483,511	$ 273,155	$ 160,163
Loss from operations	(164,227)	(128,745)	(22,147)	(78,119)	(70,448)	(74,373)	(70,854)	(57,480)	(393,238)	(273,155)	(160,163)
Net loss	$ (158,388)	$ (122,918)	$ (16,978)	$ (74,598)	$ (69,421)	$ (73,719)	$ (70,202)	$ (56,778)	$ (372,882)	$ (270,120)	$ (158,987)
Net loss per share—basic and diluted	$ (3.38)	$ (2.63)	$ (0.36)	$ (1.68)	$ (1.75)	$ (1.97)	$ (1.88)	$ (1.52)	$ (8.08)	$ (7.09)	$ (4.75)