Fair Value Measurements - Valuation of Company's Assets and Liabilities Measured at Fair Value (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Money market funds	$ 24,424	$ 99,755
Trading securities	31,274	92,642
Assets Fair Value Total	53,766	327,460
Corporate Debt [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Securities available-for-sale		137,835
Trading securities	23,237	31,120
U.S. Treasuries [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Trading securities		55,305
Mutual Funds Held In Rabbi Trust [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Trading securities	8,037	6,217
Foreign Currency Forward Position [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Foreign currency forward position	(1,932)	(2,772)
Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Money market funds	24,424	99,755
Assets Fair Value Total	24,424	99,755
Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Fair Value Total	29,342	227,705
Level 2 [Member] | Corporate Debt [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Securities available-for-sale		137,835
Trading securities	23,237	31,120
Level 2 [Member] | U.S. Treasuries [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Trading securities		55,305
Level 2 [Member] | Mutual Funds Held In Rabbi Trust [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Trading securities	8,037	6,217
Level 2 [Member] | Foreign Currency Forward Position [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Foreign currency forward position	$ (1,932)	$ (2,772)