Net Loss per Share - Computation of Basic and Diluted Net Loss per Share of Common Stock (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Earnings Per Share [Abstract]
Net loss	$ (27,689)	$ (25,826)	$ (83,988)	$ (79,250)
Weighted-average shares used to compute basic and diluted net loss per share	100,659	94,085	99,203	92,274
Net loss per share, basic and diluted (usd per share)	$ (0.28)	$ (0.27)	$ (0.85)	$ (0.86)