Fair Value Measurements - Reconciliation of Liabilities Measured at Fair Value Using Level 3 Significant Unobservable Inputs (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Balance, beginning of period		$ 2,460	$ 2,773	$ 2,773
Issuance of preferred stock warrants				868
Reclassification of stock award liability from equity upon modification			399	399
Reclassification of stock award liability to equity upon expiration				(292)
Change in fair value of stock award liability				(107)
Reclassification of preferred stock warrant liability to equity upon conversion to common stock		(521)
Change in fair value of preferred stock warrant liabilities	$ (2,097)	$ (1,939)	$ (1,408)	(1,181)
Balance, end of period				$ 2,460