Net Loss per Share - 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	$ (19,857)	$ (19,475)	$ (10,689)	$ (10,695)	$ (10,370)	$ (9,348)	$ (7,618)	$ (9,697)	$ (60,716)	$ (37,034)	$ (13,167)
Accretion on preferred stock										(86,899)	(55,886)
Net loss applicable to common stockholders									$ (60,716)	$ (123,933)	$ (69,053)
Denominator:
Weighted-average number of common shares – basic and diluted									26,469,170	15,406,386	544,002
Net loss per share applicable to common stockholders—basic and diluted	$ (0.66)	$ (0.68)	$ (0.40)	$ (0.53)	$ (0.52)	$ (0.47)	$ (0.39)	$ (43.37)	$ (2.29)	$ (8.04)	$ (126.94)