Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Assets
Total assets at fair value	$ 67,493	$ 4,800
Liabilities
Total liabilities at fair value		2,460
Preferred Stock Warrant Liability [Member]
Liabilities
Total liabilities at fair value		2,460
Level 1 [Member]
Assets
Total assets at fair value	67,493	4,800
Level 3 [Member]
Liabilities
Total liabilities at fair value		2,460
Level 3 [Member] | Preferred Stock Warrant Liability [Member]
Liabilities
Total liabilities at fair value		2,460
Cash Equivalents [Member]
Assets
Total assets at fair value	67,493	4,800
Cash Equivalents [Member] | Level 1 [Member]
Assets
Total assets at fair value	$ 67,493	$ 4,800