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	Sep. 30, 2014	Dec. 31, 2013
Level 1 [Member] | Money Market Funds [Member]
Assets:
Cash equivalents	$ 90,334	$ 47,740
Level 2 [Member] | Commercial Paper [Member]
Assets:
Cash equivalents	3,200
Investments	3,000	49,680
Level 2 [Member] | Corporate Debt Securities [Member]
Assets:
Cash equivalents		13,998
Investments	$ 50,356	$ 40,436