Fair Value Measurements - Schedule of Fair Value Financial Assets and Liabilities Measured on Recurring Basis (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Financial Assets
Total financial assets	$ 25,466	$ 37,932
Preferred Stock Warrant Liabilities [Member]
Financial Liabilities
Total financial Liabilities	10,641	1,810
Money Market Funds [Member]
Financial Assets
Total financial assets	15,213	27,847
Restricted Money Market Funds [Member]
Financial Assets
Total financial assets	225	75
Short-Term Investments [Member]
Financial Assets
Total financial assets	10,028	10,010
Level 1 [Member]
Financial Assets
Total financial assets	15,438	27,922
Level 1 [Member] | Money Market Funds [Member]
Financial Assets
Total financial assets	15,213	27,847
Level 1 [Member] | Restricted Money Market Funds [Member]
Financial Assets
Total financial assets	225	75
Level 2 [Member]
Financial Assets
Total financial assets	10,028	10,010
Level 2 [Member] | Short-Term Investments [Member]
Financial Assets
Total financial assets	10,028	10,010
Level 3 [Member] | Preferred Stock Warrant Liabilities [Member]
Financial Liabilities
Total financial Liabilities	$ 10,641	$ 1,810