Net Loss Per Share - Schedule of Computation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net loss	$ (32,849)	$ (41,708)	$ (64,911)
Weighted-average common shares outstanding	94,865	84,483	81,914
Net loss per share — basic and diluted (in dollars per share)	$ (0.35)	$ (0.49)	$ (0.79)