Fair Value Measurements - Valuation of Company's Assets and Liabilities Measured at Fair Value (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Money market funds	$ 101,896	$ 88,562
Trading securities	98,572	94,469
Assets Fair Value Total	351,225	327,381
Corporate Debt [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Securities available-for-sale	151,019	145,052
Trading securities	94,202	91,302
Mutual Funds Held In Rabbi Trust [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Trading securities	4,370	3,167
Foreign Currency Forward Position [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Foreign currency forward position	(262)	(702)
Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Money market funds	101,896	88,562
Assets Fair Value Total	101,896	88,562
Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Fair Value Total	249,329	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	151,019	145,052
Trading securities	94,202	91,302
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	4,370	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	$ (262)	$ (702)