Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule Of Investments [Line Items]
Investments	$ 591,706	$ 508,578
Commercial paper
Schedule Of Investments [Line Items]
Investments	37,157	75,362
Corporate debt securities
Schedule Of Investments [Line Items]
Investments	543,648	414,815
Domestic and foreign government agencies debt securities
Schedule Of Investments [Line Items]
Investments	$ 10,901	$ 18,401