Net Loss Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Net Loss Per Share
Net loss applicable to common stockholders (in dollars)	$ (49,520)	$ (29,044)	$ (13,510)
Weighted average number of common shares used in net loss per share applicable to common stockholders-basic and diluted	22,647	16,160	5,810
Net loss per share applicable to common stockholders-basic and diluted	$ (2.19)	$ (1.80)	$ (2.33)
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)	4,012	3,496	3,055
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)	3,683	2,509	1,410
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)	329	987	1,645