Net Loss Per Share Attributable to Common Stockholders - Reconciliation of Numerator and Denominator Used in Calculation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Numerator:
Net loss	$ (11,487)	$ (11,870)	$ (5,806)	$ (6,211)	$ (4,787)	$ (3,589)	$ (4,930)	$ (17,004)	$ (35,374)	$ (30,310)	$ (15,143)
Accretion to redemption value and cumulative dividends on preferred stock									(6,705)	(4,566)	(3,751)
Net loss attributable to common stockholders	$ (12,045)	$ (14,828)	$ (7,563)	$ (7,643)	$ (5,983)	$ (4,758)	$ (6,080)	$ (18,055)	$ (42,079)	$ (34,876)	$ (18,894)
Denominator:
Weighted-average common shares outstanding used to calculate net loss per share attributable to common stockholders, basic and diluted									8,595,247	989,453	772,320
Net loss per share attributable to common stockholders, basic and diluted									$ (4.90)	$ (35.25)	$ (24.46)