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Fair Value Measurements - Fair Values of Financial Assets Measured on Recurring Basis (Detail) - USD ($)
$ in Thousands
Mar. 31, 2020
Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments $ 414,092 $ 468,609
U.S. treasury notes and bills [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments 383,754 427,506
Bond and mutual funds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments   10,816
Equity investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments 223 255
Certificate of deposit [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments 30,115 30,032
Fair Value, Measurements, Recurring [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total fair value of financial assets 486,240 554,160
Fair Value, Measurements, Recurring [Member] | U.S. treasury notes and bills [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments 383,754 427,506
Fair Value, Measurements, Recurring [Member] | Bond and mutual funds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments   10,816
Fair Value, Measurements, Recurring [Member] | Equity investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments 223 255
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 72,148 85,551
Fair Value, Measurements, Recurring [Member] | Certificate of deposit [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments 30,115 30,032
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 375,636 444,005
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | U.S. treasury notes and bills [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments 303,265 347,383
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   10,816
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 223 255
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 72,148 85,551
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Certificate of deposit [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 110,604 110,155
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S. treasury notes and bills [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments 80,489 80,123
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
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 deposit [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments 30,115 30,032
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] | U.S. treasury notes and bills [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
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 deposit [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Investments $ 0 $ 0