Net Loss per Share - Calculation of Basic and Diluted Net Loss per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Mar. 25, 2014	Jun. 30, 2015	Jun. 30, 2014
Numerator:
Net loss	$ (10,689)	$ (7,618)		$ (21,384)	$ (17,315)
Accretion on preferred stock			$ (86,900)		(86,900)
Net loss applicable to common stockholders	$ (10,689)	$ (7,618)		$ (21,384)	$ (104,215)
Denominator:
Weighted-average number of common shares – basic and diluted	26,614,671	19,652,056		23,340,590	10,987,692
Net loss per share applicable to common stockholders—basic and diluted	$ (0.40)	$ (0.39)		$ (0.92)	$ (9.48)