Fair Value of Financial Assets and Liabilities - Unrealized Losses on Investments by Investment Category and Length of Time in Continuous Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	$ 912,073	$ 1,895,561
Less Than 12 Months, Gross Unrealized Losses	(3,216)	(14,615)
12 Months or Greater, Fair Value	96,787	85,602
12 Months or Greater, Gross Unrealized Losses	(3,951)	(8,921)
Total, Fair Value	1,008,860	1,981,163
Total, Gross Unrealized Losses	(7,167)	(23,536)
U.S. government and agency obligations
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	188,010	618,510
Less Than 12 Months, Gross Unrealized Losses	(740)	(1,434)
12 Months or Greater, Fair Value	11,930	2,107
12 Months or Greater, Gross Unrealized Losses	(227)	(54)
Total, Fair Value	199,940	620,617
Total, Gross Unrealized Losses	(967)	(1,488)
U.S. corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	265,773	449,404
Less Than 12 Months, Gross Unrealized Losses	(509)	(2,573)
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Gross Unrealized Losses	0	0
Total, Fair Value	265,773	449,404
Total, Gross Unrealized Losses	(509)	(2,573)
High yield corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	21,394	131,112
Less Than 12 Months, Gross Unrealized Losses	(475)	(7,211)
12 Months or Greater, Fair Value	598	840
12 Months or Greater, Gross Unrealized Losses	(325)	(531)
Total, Fair Value	21,992	131,952
Total, Gross Unrealized Losses	(800)	(7,742)
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	0	20,016
Less Than 12 Months, Gross Unrealized Losses	0	(24)
12 Months or Greater, Fair Value	6	5
12 Months or Greater, Gross Unrealized Losses	0	(1)
Total, Fair Value	6	20,021
Total, Gross Unrealized Losses	0	(25)
Municipal obligations
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	257,357	303,988
Less Than 12 Months, Gross Unrealized Losses	(506)	(566)
12 Months or Greater, Fair Value	12,167	8,054
12 Months or Greater, Gross Unrealized Losses	(34)	(31)
Total, Fair Value	269,524	312,042
Total, Gross Unrealized Losses	(540)	(597)
Auction rate securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	0	0
Less Than 12 Months, Gross Unrealized Losses	0	0
12 Months or Greater, Fair Value	70,086	74,596
12 Months or Greater, Gross Unrealized Losses	(3,365)	(8,304)
Total, Fair Value	70,086	74,596
Total, Gross Unrealized Losses	(3,365)	(8,304)
Foreign debt securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	178,382	372,531
Less Than 12 Months, Gross Unrealized Losses	(255)	(2,807)
12 Months or Greater, Fair Value	2,000	0
12 Months or Greater, Gross Unrealized Losses	0	0
Total, Fair Value	180,382	372,531
Total, Gross Unrealized Losses	(255)	(2,807)
Publicly traded equity securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	1,157	0
Less Than 12 Months, Gross Unrealized Losses	(731)	0
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Gross Unrealized Losses	0	0
Total, Fair Value	1,157	0
Total, Gross Unrealized Losses	$ (731)	$ 0