Fair Value Measurements - Summary of Changes in Fair Value of Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Conversion of preferred stock call option liability [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 21
Issuance of financial instruments		1,116
Fair value of call option liability recognized upon issuance of preferred stock		(126)
Change in fair value recorded in other income (expense), net	9,560	(969)
Conversion of preferred stock into common stock and reclassification to permanent equity	(9,581)
Ending balance		21
Conversion of preferred stock warrant liability [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance	474	433
Change in fair value recorded in other income (expense), net	2,278	41
Conversion of preferred stock into common stock and reclassification to permanent equity	(2,752)
Ending balance		$ 474