Fair Value Measurements - Valuation of Company's Assets and Liabilities Measured at Fair Value (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Money market funds	$ 112,529	$ 88,562
Trading securities	93,139	94,469
Assets Fair Value Total	351,613	327,381
Corporate Debt [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Securities available-for-sale	146,966	145,052
Trading securities	71,861	91,302
U.S. Treasuries [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Trading securities	17,178
Mutual Funds Held In Rabbi Trust [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Trading securities	4,100	3,167
Foreign Currency Forward Position [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Foreign currency forward position	(1,021)	(702)
Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Money market funds	112,529	88,562
Assets Fair Value Total	112,529	88,562
Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Fair Value Total	239,084	238,819
Level 2 [Member] | Corporate Debt [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Securities available-for-sale	146,966	145,052
Trading securities	71,861	91,302
Level 2 [Member] | U.S. Treasuries [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Trading securities	17,178
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	4,100	3,167
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	$ (1,021)	$ (702)