Net Income (Loss) Per Share - Calculation of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	[1]	Dec. 31, 2016	[1]
Numerator:
Net loss	$ (26,704)	$ (116,846)	[2]	$ (414,249)	[2]
Denominator:
Weighted-average shares outstanding - basic and diluted (in shares)	177,846,023	171,175,577		164,533,823
Net loss per share - basic and diluted (in USD per share)	$ (0.15)	$ (0.68)		$ (2.52)

[1] As adjusted to reflect the impact of the full retrospective adoption of Topic 606.

[2] As adjusted to reflect the impact of the full retrospective adoption of Topic 606.