Fair Value Measurements - Schedule of Fair Value Financial Assets and Liabilities Measured on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Financial Assets
Total financial assets	$ 155,991	$ 37,932
Preferred Stock Warrant Liabilities [Member]
Financial Liabilities
Total financial Liabilities		1,810
Money Market Funds [Member]
Financial Assets
Total financial assets	155,766	27,847
Restricted Money Market Funds [Member]
Financial Assets
Total financial assets	225	75
Short-Term Investments [Member]
Financial Assets
Total financial assets		10,010
Level 1 [Member]
Financial Assets
Total financial assets	155,991	27,922
Level 1 [Member] | Money Market Funds [Member]
Financial Assets
Total financial assets	155,766	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		10,010
Level 2 [Member] | Short-Term Investments [Member]
Financial Assets
Total financial assets		10,010
Level 3 [Member] | Preferred Stock Warrant Liabilities [Member]
Financial Liabilities
Total financial Liabilities		$ 1,810