Fair Value of Assets Measured on a Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, Fair value disclosure	$ 1,059,176	$ 748,984
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale equity securities, Fair value disclosure		8,997
Total	1,577,988	928,130
Recurring | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	82,262	17,199
Available for sale debt securities, Fair value disclosure	56,951	59,340
Recurring | Corporate notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	18,116
Available for sale debt securities, Fair value disclosure	373,252	333,872
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	398,298	297,773
Recurring | U.S. treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	62,855
Available for sale debt securities, Fair value disclosure	200,475	40,000
Recurring | Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	122,986	151,479
Restricted cash	1,471	1,471
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	30,200	17,999
Recurring | Quoted Prices in Active Markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale equity securities, Fair value disclosure		8,997
Total	124,457	161,947
Recurring | Quoted Prices in Active Markets (Level 1) | Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	122,986	151,479
Restricted cash	1,471	1,471
Recurring | Significant Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	1,453,531	766,183
Recurring | Significant Observable Inputs (Level 2) | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	82,262	17,199
Available for sale debt securities, Fair value disclosure	56,951	59,340
Recurring | Significant Observable Inputs (Level 2) | Corporate notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	18,116
Available for sale debt securities, Fair value disclosure	373,252	333,872
Recurring | Significant Observable Inputs (Level 2) | 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	398,298	297,773
Recurring | Significant Observable Inputs (Level 2) | U.S. treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	62,855
Available for sale debt securities, Fair value disclosure	200,475	40,000
Recurring | Significant Observable Inputs (Level 2) | Certificate of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, Fair value disclosure	$ 30,200	$ 17,999