Net Loss Per Share - Computation of Basic and Diluted Earnings Per Share (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Numerator:
Net loss									$ (36,478)	$ (14,027)	$ (13,880)
Accretion of redeemable convertible preferred stock to redemption value									(92)	(213)	(67)
Net loss attributable to common stockholders	$ (10,931)	$ (14,689)	$ (6,443)	$ (4,507)	$ (3,899)	$ (3,557)	$ (3,190)	$ (3,594)	$ (36,570)	$ (14,240)	$ (13,947)
Denominator:
Weighted average common shares outstanding, basic and diluted	22,783,817	22,707,012	2,178,465	729,391	729,391	729,391	729,391	727,763	12,189,155	729,001	709,678
Net loss per share attributable to common stockholders, basic and diluted	$ (0.48)	$ (0.65)	$ (2.96)	$ (6.18)	$ (5.35)	$ (4.88)	$ (4.37)	$ (4.94)	$ (3.00)	$ (19.53)	$ (19.65)