Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule of Investments [Line Items]
Investments	$ 283,488	$ 387,484
Certificates of deposit
Schedule of Investments [Line Items]
Investments	962	10,078
Commercial paper
Schedule of Investments [Line Items]
Investments	44,130	23,955
Corporate debt securities
Schedule of Investments [Line Items]
Investments	212,279	323,219
Domestic and foreign government agencies debt securities
Schedule of Investments [Line Items]
Investments	$ 26,117	$ 30,232