Fair Value of Financial Instruments - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (Fair Value, Measurements, Recurring [Member], USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Assets
Cash equivalents-money market fund	$ 250,882	$ 215,468
Quoted Prices in Active Markets (Level 1) [Member]
Assets
Cash equivalents-money market fund	250,882	215,468
Significant Other Observable Inputs (Level 2) [Member]
Assets
Cash equivalents-money market fund
Significant Unobservable Inputs (Level 3) [Member]
Assets
Cash equivalents-money market fund
Money Market Fund [Member]
Assets
Cash equivalents-money market fund	32,275	10,387
Money Market Fund [Member] | Quoted Prices in Active Markets (Level 1) [Member]
Assets
Cash equivalents-money market fund	32,275	10,387
Money Market Fund [Member] | Significant Other Observable Inputs (Level 2) [Member]
Assets
Cash equivalents-money market fund
Money Market Fund [Member] | Significant Unobservable Inputs (Level 3) [Member]
Assets
Cash equivalents-money market fund
US Treasury Securities Cash Equivalents [Member]
Assets
Cash equivalents-money market fund		10,011
US Treasury Securities Cash Equivalents [Member] | Quoted Prices in Active Markets (Level 1) [Member]
Assets
Cash equivalents-money market fund		10,011
US Treasury Securities Cash Equivalents [Member] | Significant Other Observable Inputs (Level 2) [Member]
Assets
Cash equivalents-money market fund
US Treasury Securities Cash Equivalents [Member] | Significant Unobservable Inputs (Level 3) [Member]
Assets
Cash equivalents-money market fund
US Treasury Securities Marketable Securities [Member]
Assets
Cash equivalents-money market fund	218,607	195,070
US Treasury Securities Marketable Securities [Member] | Quoted Prices in Active Markets (Level 1) [Member]
Assets
Cash equivalents-money market fund	218,607	195,070
US Treasury Securities Marketable Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Assets
Cash equivalents-money market fund
US Treasury Securities Marketable Securities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Assets
Cash equivalents-money market fund