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, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Level 1 [Member] | Money Market Funds [Member]
Assets:
Cash equivalents	$ 23,798	$ 33,199
Level 2 [Member] | Commercial Paper [Member]
Assets:
Investments	6,497
Level 2 [Member] | Corporate Debt Securities [Member]
Assets:
Cash equivalents		81,849
Investments	$ 56,441