Net Loss Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Earnings Per Share [Abstract]
Net loss	$ (82,260)	$ (73,219)	$ (49,520)
Weighted average number of common shares used in net loss per share applicable to common stockholders—basic and diluted (in shares)	32,148	27,085	22,647
Net loss per share applicable to common stockholders—basic and diluted (in dollars per share)	$ (2.56)	$ (2.70)	$ (2.19)
Net loss per share
Amount excluded from the calculation of diluted net loss per share, prior to the use of the treasury stock method, due to their anti-dilutive effect (in shares)	4,661	4,751	4,012
Outstanding stock options and restricted stock units
Net loss per share
Amount excluded from the calculation of diluted net loss per share, prior to the use of the treasury stock method, due to their anti-dilutive effect (in shares)	4,661	4,751	3,683
Founders' stock
Net loss per share
Amount excluded from the calculation of diluted net loss per share, prior to the use of the treasury stock method, due to their anti-dilutive effect (in shares)	0	0	329