Gross Unrealized Losses and Fair Value Available-For-Sale Investments in Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Estimated Fair Value	$ 407,946	$ 241,956
Less Than 12 Months, Unrealized Losses	(504)	(321)
12 Months or Greater, Estimated Fair Value	21,781	9,144
12 Months or Greater, Unrealized Losses	(33)	(3)
Total Estimated Fair Value	429,727	251,100
Total Unrealized Losses	(537)	(324)
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Estimated Fair Value	66,735	43,781
Less Than 12 Months, Unrealized Losses	(24)	(38)
12 Months or Greater, Estimated Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total Estimated Fair Value	66,735	43,781
Total Unrealized Losses	(24)	(38)
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Estimated Fair Value	325,223	185,243
Less Than 12 Months, Unrealized Losses	(463)	(261)
12 Months or Greater, Estimated Fair Value	14,298	9,144
12 Months or Greater, Unrealized Losses	(13)	(3)
Total Estimated Fair Value	339,521	194,387
Total Unrealized Losses	(476)	(264)
Domestic and foreign government agencies debt securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Estimated Fair Value	15,988	12,932
Less Than 12 Months, Unrealized Losses	(17)	(22)
12 Months or Greater, Estimated Fair Value	7,483	0
12 Months or Greater, Unrealized Losses	(20)	0
Total Estimated Fair Value	23,471	12,932
Total Unrealized Losses	$ (37)	$ (22)