Net Loss Per Share - Summary of Basic and Diluted Net Loss Per Share (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
Numerator:
Net loss	$ (168,653)	$ (179,958)	$ (168,221)	$ (163,406)	$ (158,388)	$ (122,918)	$ (16,978)	$ (74,598)	$ (680,238)	$ (372,882)	$ (270,120)
Denominator:
Weighted average common stock outstanding —basic									50,833,837	46,121,194	38,113,678
Dilutive effect of shares of common stock equivalents resulting from common stock options and restricted stock units									0	0	0
Weighted average common stock outstanding —diluted									50,833,837	46,121,194	38,113,678
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)