Fair Value of Assets Measured on a Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, Fair value disclosure	$ 1,172,363	$ 1,059,176
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	1,436,331	1,577,988
Recurring | Quoted Prices in Active Markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	233,328	124,457
Recurring | Significant Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	1,203,003	1,453,531
Recurring | Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	231,854	122,986
Restricted cash	1,474	1,471
Recurring | Money Market Funds | Quoted Prices in Active Markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	231,854	122,986
Restricted cash	1,474	1,471
Recurring | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	19,981	82,262
Available for sale debt securities, Fair value disclosure	96,539	56,951
Recurring | Commercial paper | Significant Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	19,981	82,262
Available for sale debt securities, Fair value disclosure	96,539	56,951
Recurring | Corporate notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	5,168	18,116
Available for sale debt securities, Fair value disclosure	401,035	373,252
Recurring | Corporate notes | Significant Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	5,168	18,116
Available for sale debt securities, Fair value disclosure	401,035	373,252
Recurring | U.S. treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	5,491	62,855
Available for sale debt securities, Fair value disclosure	262,387	200,475
Recurring | U.S. treasury securities | Significant Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	5,491	62,855
Available for sale debt securities, Fair value disclosure	262,387	200,475
Recurring | Certificate of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, Fair value disclosure	33,541	30,200
Recurring | Certificate of deposit | Significant Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, Fair value disclosure	33,541	30,200
Recurring | U.S. government-sponsored enterprise securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		231,122
Available for sale debt securities, Fair value disclosure	378,861	398,298
Recurring | U.S. government-sponsored enterprise securities | Significant Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		231,122
Available for sale debt securities, Fair value disclosure	$ 378,861	$ 398,298