Unrealized Losses on Investments by Investment Category and Length of Time in Continuous Unrealized Loss Position (Detail) (Debt Securities, USD $)	Sep. 30, 2011	Dec. 31, 2010
Less Than 12 Months Fair Value	$ 2,241,952,000
Less Than 12 Months Gross Unrealized Losses	(36,356,000)
12 Months or Greater Fair Value	95,734,000
12 Months or Greater Gross Unrealized Losses	(10,413,000)
Tota Fair Value	2,337,686,000
Total Gross Unrealized Losses	(46,769,000)	(19,811,000)
US Government Agencies Debt Securities
Less Than 12 Months Fair Value	747,809,000
Less Than 12 Months Gross Unrealized Losses	(2,122,000)
12 Months or Greater Fair Value	6,491,000
12 Months or Greater Gross Unrealized Losses	(146,000)
Tota Fair Value	754,300,000
Total Gross Unrealized Losses	(2,268,000)	(2,674,000)
US Corporate Debt Securities
Less Than 12 Months Fair Value	549,667,000
Less Than 12 Months Gross Unrealized Losses	(3,595,000)
Tota Fair Value	549,667,000
Total Gross Unrealized Losses	(3,595,000)	(1,307,000)
High Yield Corporate Debt Securities
Less Than 12 Months Fair Value	306,934,000
Less Than 12 Months Gross Unrealized Losses	(27,557,000)
Tota Fair Value	306,934,000
Total Gross Unrealized Losses	(27,557,000)	(1,943,000)
Asset-backed Securities
Less Than 12 Months Fair Value	7,047,000
Less Than 12 Months Gross Unrealized Losses	(10,000)
12 Months or Greater Fair Value	5,000
12 Months or Greater Gross Unrealized Losses	(1,000)
Tota Fair Value	7,052,000
Total Gross Unrealized Losses	(11,000)	(1,000)
Municipal Obligations
Less Than 12 Months Fair Value	304,076,000
Less Than 12 Months Gross Unrealized Losses	(472,000)
12 Months or Greater Fair Value	8,147,000
12 Months or Greater Gross Unrealized Losses	(7,000)
Tota Fair Value	312,223,000
Total Gross Unrealized Losses	(479,000)	(3,266,000)
Auction Rate Securities
12 Months or Greater Fair Value	81,091,000
12 Months or Greater Gross Unrealized Losses	(10,259,000)
Tota Fair Value	81,091,000
Total Gross Unrealized Losses	(10,259,000)	(9,906,000)
Foreign Debt Securities
Less Than 12 Months Fair Value	326,419,000
Less Than 12 Months Gross Unrealized Losses	(2,600,000)
12 Months or Greater Fair Value	0
12 Months or Greater Gross Unrealized Losses	0
Tota Fair Value	326,419,000
Total Gross Unrealized Losses	$ (2,600,000)	$ (714,000)