Gross Unrealized Losses and Fair Value Available-For-Sale Investments in Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Less Than 12 Months, Estimated Fair Value	$ 534,736	$ 460,252
Less Than 12 Months, Unrealized Losses	(3,618)	(1,777)
12 Months or Greater, Estimated Fair Value	26,751	35,566
12 Months or Greater, Unrealized Losses	(79)	(74)
Total Estimated Fair Value	561,487	495,818
Total Unrealized Losses	(3,697)	(1,851)
Commercial paper
Schedule Of Available For Sale Securities [Line Items]
Less Than 12 Months, Estimated Fair Value	41,993	62,602
Less Than 12 Months, Unrealized Losses	(112)	(35)
12 Months or Greater, Estimated Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total Estimated Fair Value	41,993	62,602
Total Unrealized Losses	(112)	(35)
Corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Less Than 12 Months, Estimated Fair Value	481,836	386,728
Less Than 12 Months, Unrealized Losses	(3,409)	(1,660)
12 Months or Greater, Estimated Fair Value	19,267	28,087
12 Months or Greater, Unrealized Losses	(62)	(50)
Total Estimated Fair Value	501,103	414,815
Total Unrealized Losses	(3,471)	(1,710)
Domestic and foreign government agencies debt securities
Schedule Of Available For Sale Securities [Line Items]
Less Than 12 Months, Estimated Fair Value	10,907	10,922
Less Than 12 Months, Unrealized Losses	(97)	(82)
12 Months or Greater, Estimated Fair Value	7,484	7,479
12 Months or Greater, Unrealized Losses	(17)	(24)
Total Estimated Fair Value	18,391	18,401
Total Unrealized Losses	$ (114)	$ (106)