Unrealized Losses on Investments by Investment Category and Length of Time in Continuous Unrealized Loss Position (Detail) (Debt Securities, USD $)	Mar. 31, 2012	Dec. 31, 2011
Less Than 12 Months Fair Value	$ 1,650,329,000
Less Than 12 Months Gross Unrealized Losses	(5,931,000)
12 Months or Greater Fair Value	81,573,000
12 Months or Greater Gross Unrealized Losses	(4,689,000)
Total Fair Value	1,731,902,000
Total Gross Unrealized Losses	(10,620,000)	(23,536,000)
US Government Agencies Debt Securities
Less Than 12 Months Fair Value	862,747,000
Less Than 12 Months Gross Unrealized Losses	(1,651,000)
12 Months or Greater Fair Value	1,482,000
12 Months or Greater Gross Unrealized Losses	(29,000)
Total Fair Value	864,229,000
Total Gross Unrealized Losses	(1,680,000)	(1,488,000)
US Corporate Debt Securities
Less Than 12 Months Fair Value	254,514,000
Less Than 12 Months Gross Unrealized Losses	(475,000)
Total Fair Value	254,514,000
Total Gross Unrealized Losses	(475,000)	(2,573,000)
High Yield Corporate Debt Securities
Less Than 12 Months Fair Value	64,144,000
Less Than 12 Months Gross Unrealized Losses	(2,818,000)
12 Months or Greater Fair Value	1,689,000
12 Months or Greater Gross Unrealized Losses	(381,000)
Total Fair Value	65,833,000
Total Gross Unrealized Losses	(3,199,000)	(7,742,000)
Asset-backed Securities
Less Than 12 Months Fair Value	12,391,000
Less Than 12 Months Gross Unrealized Losses	(14,000)
12 Months or Greater Fair Value	5,000
12 Months or Greater Gross Unrealized Losses	(1,000)
Total Fair Value	12,396,000
Total Gross Unrealized Losses	(15,000)	(25,000)
Municipal Obligations
Less Than 12 Months Fair Value	287,508,000
Less Than 12 Months Gross Unrealized Losses	(655,000)
Total Fair Value	287,508,000
Total Gross Unrealized Losses	(655,000)	(597,000)
Auction Rate Securities
12 Months or Greater Fair Value	78,397,000
12 Months or Greater Gross Unrealized Losses	(4,278,000)
Total Fair Value	78,397,000
Total Gross Unrealized Losses	(4,278,000)	(8,304,000)
Foreign Debt Securities
Less Than 12 Months Fair Value	169,025,000
Less Than 12 Months Gross Unrealized Losses	(318,000)
Total Fair Value	169,025,000
Total Gross Unrealized Losses	$ (318,000)	$ (2,807,000)