Fair Value Measurements - Summary of Changes in Fair Value of Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended		70 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014
Conversion of preferred stock call option liability [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 21	$ 0
Issuance of financial instruments		990
Fair value of call option liability recognized upon issuance of preferred stock	0	(127)
Change in fair value recorded in other income (expense), net	9,560	(863)
Conversion of preferred stock into common stock and reclassification to permanent equity	(9,581)		(9,581)
Ending balance	0	0	0
Conversion of preferred stock warrant liability [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance	474	433
Fair value of call option liability recognized upon issuance of preferred stock	0	0
Change in fair value recorded in other income (expense), net	2,279	(1)
Conversion of preferred stock into common stock and reclassification to permanent equity	(2,753)		(2,753)
Ending balance	$ 0	$ 432	$ 0