Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule Of Investments [Line Items]
Investments	$ 561,487	$ 508,578
Commercial paper
Schedule Of Investments [Line Items]
Investments	41,993	75,362
Corporate debt securities
Schedule Of Investments [Line Items]
Investments	501,103	414,815
Domestic and foreign government agencies debt securities
Schedule Of Investments [Line Items]
Investments	$ 18,391	$ 18,401