Investments in Fixed Maturity Securities (Investments in Fixed Maturity Securities) (Details) (USD $)	Dec. 31, 2012		Dec. 31, 2011
Total held to maturity, Amortized Cost	$ 102,951,000		$ 115,109,000
Total held to maturity, Gross Unrealized Gains	24,420,000		18,873,000
Total held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	127,371,000		133,982,000
Total held to maturity, Carrying Value	102,951,000		115,109,000
Total available for sale, Amortized Cost	11,115,003,000		10,639,047,000
Total available for sale, Gross Unrealized Gains	758,662,000		632,430,000
Total available for sale, Gross Unrealized Losses	(32,660,000)		(74,549,000)
Total available for sale, Fair Value	11,841,005,000		11,196,928,000
Total available for sale, Carrying Value	11,841,005,000		11,196,928,000
Total investment in fixed maturity securities, Amortized Cost	11,217,954,000		10,754,156,000
Total investments in fixed maturity securities, Gross Unrealized Gains	783,082,000		651,303,000
Total investments in fixed maturity securities, Gross Unrealized Losses	(32,660,000)		(74,549,000)
Total investments in fixed maturity securities, Fair Value	11,968,376,000		11,330,910,000
Total investments in fixed maturity securities, carrying value	11,943,956,000		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	827,591,000		906,924,000
Total available for sale, Gross Unrealized Gains	72,532,000		69,920,000
Total available for sale, Gross Unrealized Losses	(1,660,000)		(351,000)
Total available for sale, Fair Value	898,463,000		976,493,000
Total available for sale, Carrying Value	898,463,000		976,493,000
State and Local Jurisdiction [Member]
Total available for sale, Amortized Cost	4,449,238,000		5,031,275,000
Total available for sale, Gross Unrealized Gains	328,974,000		308,345,000
Total available for sale, Gross Unrealized Losses	(9,693,000)		(16,550,000)
Total available for sale, Fair Value	4,768,519,000		5,323,070,000
Total available for sale, Carrying Value	4,768,519,000		5,323,070,000
Residential Mortgage Backed Securities [Member]
Total available for sale, Amortized Cost	1,395,739,000	[1]	1,416,427,000	[1]
Total available for sale, Gross Unrealized Gains	53,846,000	[1]	75,635,000	[1]
Total available for sale, Gross Unrealized Losses	(7,456,000)	[1]	(15,894,000)	[1]
Total available for sale, Fair Value	1,442,129,000	[1]	1,476,168,000	[1]
Total available for sale, Carrying Value	1,442,129,000	[1]	1,476,168,000	[1]
Non-credit portion of other-than-temporary impairments ("OTTI")	3,037,000		7,668,000
Commercial Mortgage-Backed Securities [Member]
Total available for sale, Amortized Cost	268,671,000		105,383,000
Total available for sale, Gross Unrealized Gains	5,641,000		4,054,000
Total available for sale, Gross Unrealized Losses	(744,000)		(1,018,000)
Total available for sale, Fair Value	273,568,000		108,419,000
Total available for sale, Carrying Value	273,568,000		108,419,000
Corporate Debt Securities [Member]
Total available for sale, Amortized Cost	3,144,480,000		2,328,200,000
Total available for sale, Gross Unrealized Gains	214,322,000		132,311,000
Total available for sale, Gross Unrealized Losses	(12,083,000)		(36,087,000)
Total available for sale, Fair Value	3,346,719,000		2,424,424,000
Total available for sale, Carrying Value	3,346,719,000		2,424,424,000
Foreign [Member]
Total available for sale, Amortized Cost	1,029,284,000		850,838,000
Total available for sale, Gross Unrealized Gains	83,347,000		42,165,000
Total available for sale, Gross Unrealized Losses	(1,024,000)		(4,649,000)
Total available for sale, Fair Value	1,111,607,000		888,354,000
Total available for sale, Carrying Value	1,111,607,000		888,354,000
State and Local Jurisdiction [Member]
Total held to maturity, Amortized Cost	65,190,000		74,354,000
Total held to maturity, Gross Unrealized Gains	18,529,000		12,546,000
Total held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	83,719,000		86,900,000
Total held to maturity, Carrying Value	65,190,000		74,354,000
Residential Mortgage Backed Securities [Member]
Total held to maturity, Amortized Cost	32,764,000		35,759,000
Total held to maturity, Gross Unrealized Gains	5,286,000		5,610,000
Total held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	38,050,000		41,369,000
Total held to maturity, Carrying Value	32,764,000		35,759,000
Corporate Debt Securities [Member]
Total held to maturity, Amortized Cost	4,997,000		4,996,000
Total held to maturity, Gross Unrealized Gains	605,000		717,000
Total held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	5,602,000		5,713,000
Total held to maturity, Carrying Value	$ 4,997,000		$ 4,996,000

[1] Gross unrealized losses for mortgage-backed securities include $3,037,000 and $7,668,000, as of December 31, 2012 and 2011, respectively, related to the non-credit portion of OTTI recognized in other comprehensive income.