Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Investments	$ 453,458	$ 267,573
Certificates of deposit
Schedule of Investments [Line Items]
Investments	240	960
Commercial paper
Schedule of Investments [Line Items]
Investments	66,735	49,245
Corporate debt securities
Schedule of Investments [Line Items]
Investments	363,012	204,436
Domestic and foreign government agencies debt securities
Schedule of Investments [Line Items]
Investments	$ 23,471	$ 12,932