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Fair Value Measurements - Fair Values of Financial Assets Measured on Recurring Basis (Detail) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments $ 95,366 $ 150,407
Corporate bonds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments 75,872 126,683
Bond and mutual funds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments 18,251 22,462
Equity investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments 207 225
Certificate of deposits [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments 1,036 1,037
Fair Value, Measurements, Recurring [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total fair value of financial assets 641,864 244,950
Fair Value, Measurements, Recurring [Member] | Corporate bonds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments 75,872 126,683
Fair Value, Measurements, Recurring [Member] | Bond and mutual funds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments 18,251 22,462
Fair Value, Measurements, Recurring [Member] | Equity investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments 207 225
Fair Value, Measurements, Recurring [Member] | Money market funds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Assets fair value disclosure 546,498 94,543
Fair Value, Measurements, Recurring [Member] | Certificate of deposits [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments 1,036 1,037
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total fair value of financial assets 564,956 117,230
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Corporate bonds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments 0 0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Bond and mutual funds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments 18,251 22,462
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Equity investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments 207 225
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Money market funds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Assets fair value disclosure 546,498 94,543
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Certificate of deposits [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments 0 0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total fair value of financial assets 76,908 127,720
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Corporate bonds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments 75,872 126,683
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Bond and mutual funds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments 0 0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Equity investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments 0 0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Money market funds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Assets fair value disclosure 0 0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Certificate of deposits [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments 1,036 1,037
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total fair value of financial assets 0 0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Corporate bonds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments 0 0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Bond and mutual funds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments 0 0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Equity investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments 0 0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Money market funds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Assets fair value disclosure 0 0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Certificate of deposits [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments $ 0 $ 0