Gross Unrealized Losses and Fair Value Available-For-Sale Investments in Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Estimated Fair Value	$ 358,549	$ 167,462
Less Than 12 Months, Unrealized Losses	(959)	(283)
12 Months or Greater, Estimated Fair Value	5,521	0
12 Months or Greater, Unrealized Losses	(22)	0
Total Estimated Fair Value	364,070	167,462
Total Unrealized Losses	(981)	(283)
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Estimated Fair Value	5,517	16,957
Less Than 12 Months, Unrealized Losses	(3)	(34)
12 Months or Greater, Estimated Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total Estimated Fair Value	5,517	16,957
Total Unrealized Losses	(3)	(34)
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Estimated Fair Value	16,959
Less Than 12 Months, Unrealized Losses	(11)
12 Months or Greater, Estimated Fair Value	0
12 Months or Greater, Unrealized Losses	0
Total Estimated Fair Value	16,959
Total Unrealized Losses	(11)
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Estimated Fair Value	310,160	149,477
Less Than 12 Months, Unrealized Losses	(880)	(248)
12 Months or Greater, Estimated Fair Value	5,521	0
12 Months or Greater, Unrealized Losses	(22)	0
Total Estimated Fair Value	315,681	149,477
Total Unrealized Losses	(902)	(248)
Securities of government sponsored entities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Estimated Fair Value	25,913	1,028
Less Than 12 Months, Unrealized Losses	(65)	(1)
12 Months or Greater, Estimated Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total Estimated Fair Value	25,913	1,028
Total Unrealized Losses	$ (65)	$ (1)