Net Loss per Share - Calculation of Basic and Diluted Net Loss Per Share (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	$ (76,429)	$ (60,940)	$ (171,843)	$ (150,419)
Weighted average common shares used for basic and diluted net loss per share computation (in shares)	87,283	85,105	86,679	84,779
Net loss per share:
Basic and Diluted (in dollars per share)	$ (0.88)	$ (0.72)	$ (1.98)	$ (1.77)