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Fair Value Measurements - Fair Values of Financial Assets Measured on Recurring Basis (Detail) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments $ 38,681 $ 72,566
Corporate bonds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments 28,094 53,943
Bond and mutual funds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments 10,377 18,402
Equity investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments 210 221
Fair Value, Measurements, Recurring [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total fair value of financial assets 618,968 642,508
Fair Value, Measurements, Recurring [Member] | Corporate bonds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments 28,094 53,943
Fair Value, Measurements, Recurring [Member] | Bond and mutual funds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments 10,377 18,402
Fair Value, Measurements, Recurring [Member] | Equity investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments 210 221
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 580,287 569,942
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 590,874 588,565
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 10,377 18,402
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 210 221
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 580,287 569,942
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 28,094 53,943
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 28,094 53,943
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 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