Unrealized Losses on Investments by Investment Category and Length of Time in Continuous Unrealized Loss Position (Detail) (Debt Securities, USD $)	Dec. 31, 2011	Dec. 31, 2010
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Less Than 12 Months Fair Value	$ 1,895,561,000	$ 1,561,337,000
Less Than 12 Months Gross Unrealized Losses	(14,615,000)	(9,799,000)
12 Months or Greater Fair Value	85,602,000	151,523,000
12 Months or Greater Gross Unrealized Losses	(8,921,000)	(10,012,000)
Total Fair Value	1,981,163,000	1,712,860,000
Total Gross Unrealized Losses	(23,536,000)	(19,811,000)
US Government Agencies Debt Securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Less Than 12 Months Fair Value	618,510,000	491,897,000
Less Than 12 Months Gross Unrealized Losses	(1,434,000)	(2,569,000)
12 Months or Greater Fair Value	2,107,000	5,474,000
12 Months or Greater Gross Unrealized Losses	(54,000)	(105,000)
Total Fair Value	620,617,000	497,371,000
Total Gross Unrealized Losses	(1,488,000)	(2,674,000)
US Corporate Debt Securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Less Than 12 Months Fair Value	449,404,000	291,157,000
Less Than 12 Months Gross Unrealized Losses	(2,573,000)	(1,307,000)
Total Fair Value	449,404,000	291,157,000
Total Gross Unrealized Losses	(2,573,000)	(1,307,000)
High Yield Corporate Debt Securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Less Than 12 Months Fair Value	131,112,000	66,537,000
Less Than 12 Months Gross Unrealized Losses	(7,211,000)	(1,943,000)
12 Months or Greater Fair Value	840,000
12 Months or Greater Gross Unrealized Losses	(531,000)
Total Fair Value	131,952,000	66,537,000
Total Gross Unrealized Losses	(7,742,000)	(1,943,000)
Asset-backed Securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Less Than 12 Months Fair Value	20,016,000	6,998,000
Less Than 12 Months Gross Unrealized Losses	(24,000)	0
12 Months or Greater Fair Value	5,000	5,000
12 Months or Greater Gross Unrealized Losses	(1,000)	(1,000)
Total Fair Value	20,021,000	7,003,000
Total Gross Unrealized Losses	(25,000)	(1,000)
Municipal Obligations
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Less Than 12 Months Fair Value	303,988,000	599,814,000
Less Than 12 Months Gross Unrealized Losses	(566,000)	(3,266,000)
12 Months or Greater Fair Value	8,054,000
12 Months or Greater Gross Unrealized Losses	(31,000)
Total Fair Value	312,042,000	599,814,000
Total Gross Unrealized Losses	(597,000)	(3,266,000)
Auction Rate Securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
12 Months or Greater Fair Value	74,596,000	146,044,000
12 Months or Greater Gross Unrealized Losses	(8,304,000)	(9,906,000)
Total Fair Value	74,596,000	146,044,000
Total Gross Unrealized Losses	(8,304,000)	(9,906,000)
Foreign Debt Securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Less Than 12 Months Fair Value	372,531,000	104,934,000
Less Than 12 Months Gross Unrealized Losses	(2,807,000)	(714,000)
Total Fair Value	372,531,000	104,934,000
Total Gross Unrealized Losses	$ (2,807,000)	$ (714,000)