Fair Value of Financial Instruments - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Assets
Marketable securities	$ 422,887	$ 346,596
Assets	563,674	395,516
Money market fund [Member]
Assets
Cash equivalents-money market fund	111,778	27,899
US treasury securities Cash equivalents [Member]
Assets
Cash equivalents-money market fund	29,009	21,021
US treasury securities Marketable securities [Member]
Assets
Marketable securities	422,887	346,596
Quoted Price in Active Markets (Level 1) [Member]
Assets
Assets	111,778	27,899
Quoted Price in Active Markets (Level 1) [Member] | Money market fund [Member]
Assets
Cash equivalents-money market fund	111,778	27,899
Significant Other Observable Inputs (Level 2) [Member]
Assets
Assets	451,896	367,617
Significant Other Observable Inputs (Level 2) [Member] | US treasury securities Cash equivalents [Member]
Assets
Cash equivalents-money market fund	29,009	21,021
Significant Other Observable Inputs (Level 2) [Member] | US treasury securities Marketable securities [Member]
Assets
Marketable securities	422,887	346,596
Significant Unobservable Inputs (Level 3) [Member]
Assets
Assets
Significant Unobservable Inputs (Level 3) [Member] | Money market fund [Member]
Assets
Cash equivalents-money market fund
Significant Unobservable Inputs (Level 3) [Member] | US treasury securities Cash equivalents [Member]
Assets
Cash equivalents-money market fund
Significant Unobservable Inputs (Level 3) [Member] | US treasury securities Marketable securities [Member]
Assets
Marketable securities