Investments in Fixed Maturity Securities (Investments in Fixed Maturity Securities) (Details) (USD $)	Mar. 31, 2012		Dec. 31, 2011
Total held to maturity, Amortized Cost	$ 115,692,000		$ 115,109,000
Total held to maturity, Gross Unrealized Gains	18,531,000		18,873,000
Total held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	134,223,000		133,982,000
Total held to maturity, Carrying Value	115,692,000		115,109,000
Total available for sale, Amortized Cost	10,858,201,000		10,639,047,000
Total available for sale, Gross Unrealized Gains	637,184,000		632,430,000
Total available for sale, Gross Unrealized Losses	(56,868,000)		(74,549,000)
Total available for sale, Fair Value	11,438,517,000		11,196,928,000
Total available for sale, Carrying Value	11,438,517,000		11,196,928,000
Total investment in fixed maturity securities, Amortized Cost	10,973,893,000		10,754,156,000
Total investments in fixed maturity securities, Gross Unrealized Gains	655,715,000		651,303,000
Total investments in fixed maturity securities, Gross Unrealized Losses	(56,868,000)		(74,549,000)
Total investments in fixed maturity securities, Fair Value	11,572,740,000		11,330,910,000
Total investments in fixed maturity securities, carrying value	11,554,209,000		11,312,037,000
Minimum ownership of Common stockholders equity	10.00%
U.S. Government And Government Agency [Member]
Total available for sale, Amortized Cost	951,785,000		906,924,000
Total available for sale, Gross Unrealized Gains	74,897,000		69,920,000
Total available for sale, Gross Unrealized Losses	(420,000)		(351,000)
Total available for sale, Fair Value	1,026,262,000		976,493,000
Total available for sale, Carrying Value	1,026,262,000		976,493,000
State And Municipal [Member]
Total available for sale, Amortized Cost	4,971,415,000		5,031,275,000
Total available for sale, Gross Unrealized Gains	308,993,000		308,345,000
Total available for sale, Gross Unrealized Losses	(18,600,000)		(16,550,000)
Total available for sale, Fair Value	5,261,808,000		5,323,070,000
Total available for sale, Carrying Value	5,261,808,000		5,323,070,000
Residential Mortgage-Backed [Member]
Total available for sale, Amortized Cost	1,380,703,000	[1]	1,416,427,000	[1]
Total available for sale, Gross Unrealized Gains	55,984,000	[1]	75,635,000	[1]
Total available for sale, Gross Unrealized Losses	(11,365,000)	[1]	(15,894,000)	[1]
Total available for sale, Fair Value	1,425,322,000	[1]	1,476,168,000	[1]
Total available for sale, Carrying Value	1,425,322,000	[1]	1,476,168,000	[1]
Non-credit portion of other-than-temporary impairments ("OTTI")	5,926,000		7,668,000
Commercial Mortgage-Backed Securities [Member]
Total available for sale, Amortized Cost	103,852,000		105,383,000
Total available for sale, Gross Unrealized Gains	6,185,000		4,054,000
Total available for sale, Gross Unrealized Losses	(1,185,000)		(1,018,000)
Total available for sale, Fair Value	108,852,000		108,419,000
Total available for sale, Carrying Value	108,852,000		108,419,000
Corporate [Member]
Total available for sale, Amortized Cost	2,487,439,000		2,328,200,000
Total available for sale, Gross Unrealized Gains	146,865,000		132,311,000
Total available for sale, Gross Unrealized Losses	(24,024,000)		(36,087,000)
Total available for sale, Fair Value	2,610,280,000		2,424,424,000
Total available for sale, Carrying Value	2,610,280,000		2,424,424,000
Foreign [Member]
Total available for sale, Amortized Cost	963,007,000		850,838,000
Total available for sale, Gross Unrealized Gains	44,260,000		42,165,000
Total available for sale, Gross Unrealized Losses	(1,274,000)		(4,649,000)
Total available for sale, Fair Value	1,005,993,000		888,354,000
Total available for sale, Carrying Value	1,005,993,000		888,354,000
State And Municipal [Member]
Total held to maturity, Amortized Cost	75,473,000		74,354,000
Total held to maturity, Gross Unrealized Gains	12,129,000		12,546,000
Total held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	87,602,000		86,900,000
Total held to maturity, Carrying Value	75,473,000		74,354,000
Residential Mortgage-Backed [Member]
Total held to maturity, Amortized Cost	35,223,000		35,759,000
Total held to maturity, Gross Unrealized Gains	5,757,000		5,610,000
Total held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	40,980,000		41,369,000
Total held to maturity, Carrying Value	35,223,000		35,759,000
Corporate [Member]
Total held to maturity, Amortized Cost	4,996,000		4,996,000
Total held to maturity, Gross Unrealized Gains	645,000		717,000
Total held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	5,641,000		5,713,000
Total held to maturity, Carrying Value	$ 4,996,000		$ 4,996,000

[1] Gross unrealized losses for residential mortgage-backed securities include $5,926,000 and $7,668,000 as of March 31, 2012 and December 31, 2011, respectively, related to the non-credit portion of other-than-temporary impairments (“OTTI”) recognized in other comprehensive income.