Fair Value of Financial Instruments - Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Detail) - Recurring Basis [Member] - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Level 1 [Member] | Money Market Funds [Member]
Assets:
Cash equivalents	$ 15,187	$ 33,199
Level 2 [Member] | Commercial Paper [Member]
Assets:
Investments	5,000	6,491
Level 2 [Member] | Corporate Debt Securities [Member]
Assets:
Investments	$ 28,991	$ 81,849