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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net loss	$ (69,421)	$ (73,719)	$ (70,202)	$ (56,778)	$ (55,901)	$ (37,796)	$ (34,747)	$ (30,543)	$ (270,120)	$ (158,987)	$ (94,495)
Denominator:
Weighted average common stock outstanding—basic									38,113,678	33,492,795	27,778,288
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									38,113,678	33,492,795	27,778,288
Net loss per share—basic and diluted	$ (1.75)	$ (1.97)	$ (1.88)	$ (1.52)	$ (1.50)	$ (1.15)	$ (1.08)	$ (0.97)	$ (7.09)	$ (4.75)	$ (3.40)