Investments in Fixed Maturity Securities (Investments in Fixed Maturity Securities) (Details) (USD $)	Sep. 30, 2011	Dec. 31, 2010
Total held to maturity, Amortized Cost	$ 114,778,000	$ 115,995,000
Total held to maturity, Gross Unrealized Gains	17,727,000	7,292,000
Total held to maturity, Gross Unrealized Losses	0	(1,129,000)
Total held to maturity, Fair Value	132,505,000	122,158,000
Total held to maturity, Carrying Value	114,778,000	115,995,000
Total available for sale, Amortized Cost	10,578,463,000	10,737,198,000
Total available for sale, Gross Unrealized Gains	610,102,000	449,840,000
Total available for sale, Gross Unrealized Losses	(72,573,000)	(93,879,000)
Total available for sale, Fair Value	11,115,992,000	11,093,159,000
Total available for sale, Carrying Value	11,115,992,000	11,093,159,000
Total investment in fixed maturity securities, Amortized Cost	10,693,241,000	10,853,193,000
Total investments in fixed maturity securities, Gross Unrealized Gains	627,829,000	457,132,000
Total investments in fixed maturity securities, Gross Unrealized Losses	(72,573,000)	(95,008,000)
Total investments in fixed maturity securities, Fair Value	11,248,497,000	11,215,317,000
Total investments in fixed maturity securities, carrying value	11,230,770,000	11,209,154,000
Minimum ownership of Common stockholders equity	10.00%
State And Municipal [Member]
Total held to maturity, Amortized Cost	73,282,000	71,998,000
Total held to maturity, Gross Unrealized Gains	10,920,000	3,440,000
Total held to maturity, Gross Unrealized Losses	0	(1,129,000)
Total held to maturity, Fair Value	84,202,000	74,309,000
Total held to maturity, Carrying Value	73,282,000	71,998,000
State And Municipal [Member]
Total available for sale, Amortized Cost	5,019,033,000	5,302,513,000
Total available for sale, Gross Unrealized Gains	286,782,000	203,221,000
Total available for sale, Gross Unrealized Losses	(19,277,000)	(44,288,000)
Total available for sale, Fair Value	5,286,538,000	5,461,446,000
Total available for sale, Carrying Value	5,286,538,000	5,461,446,000
Residential Mortgage-Backed [Member]
Total held to maturity, Amortized Cost	36,500,000	39,002,000
Total held to maturity, Gross Unrealized Gains	6,061,000	3,667,000
Total held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	42,561,000	42,669,000
Total held to maturity, Carrying Value	36,500,000	39,002,000
Residential Mortgage-Backed [Member]
Total available for sale, Amortized Cost	1,383,414,000	1,319,289,000	[1]
Total available for sale, Gross Unrealized Gains	77,663,000	52,165,000	[1]
Total available for sale, Gross Unrealized Losses	(14,067,000)	(13,278,000)	[1]
Total available for sale, Fair Value	1,447,010,000	1,358,176,000	[1]
Total available for sale, Carrying Value	1,447,010,000	1,358,176,000	[1]
Non-credit portion of other-than-temporary impairments ("OTTI")	6,020,000	4,064,000
Corporate [Member]
Total held to maturity, Amortized Cost	4,996,000	4,995,000
Total held to maturity, Gross Unrealized Gains	746,000	185,000
Total held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	5,742,000	5,180,000
Total held to maturity, Carrying Value	4,996,000	4,995,000
Corporate [Member]
Total available for sale, Amortized Cost	2,613,879,000	2,307,987,000
Total available for sale, Gross Unrealized Gains	136,728,000	102,306,000
Total available for sale, Gross Unrealized Losses	(35,156,000)	(30,031,000)
Total available for sale, Fair Value	2,715,451,000	2,380,262,000
Total available for sale, Carrying Value	2,715,451,000	2,380,262,000
U.S. Government And Government Agency [Member]
Total available for sale, Amortized Cost	995,463,000	1,289,669,000
Total available for sale, Gross Unrealized Gains	78,618,000	58,658,000
Total available for sale, Gross Unrealized Losses	(344,000)	(452,000)
Total available for sale, Fair Value	1,073,737,000	1,347,875,000
Total available for sale, Carrying Value	1,073,737,000	1,347,875,000
Commercial Mortgage-Backed Securities [Member]
Total available for sale, Amortized Cost	53,966,000	57,057,000
Total available for sale, Gross Unrealized Gains	2,791,000	2,207,000
Total available for sale, Gross Unrealized Losses	(1,480,000)	(5,594,000)
Total available for sale, Fair Value	55,277,000	53,670,000
Total available for sale, Carrying Value	55,277,000	53,670,000
Foreign [Member]
Total available for sale, Amortized Cost	512,708,000	460,683,000
Total available for sale, Gross Unrealized Gains	27,520,000	31,283,000
Total available for sale, Gross Unrealized Losses	(2,249,000)	(236,000)
Total available for sale, Fair Value	537,979,000	491,730,000
Total available for sale, Carrying Value	$ 537,979,000	$ 491,730,000

[1] Gross unrealized losses for residential mortgage-backed securities include $6,020,000 and $4,064,000 as of September 30, 2011 and December 31, 2010, respectively, related to the non-credit portion of other-than-temporary impairments (“OTTI”) recognized in other comprehensive income.