Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Schedule of Investments [Line Items]
Investments	$ 328,330	$ 387,484
Certificates of deposit
Schedule of Investments [Line Items]
Investments	4,563	10,078
Commercial paper
Schedule of Investments [Line Items]
Investments	30,702	23,955
Corporate debt securities
Schedule of Investments [Line Items]
Investments	261,855	323,219
Domestic and foreign government agencies debt securities
Schedule of Investments [Line Items]
Investments	$ 31,210	$ 30,232