Investments in Fixed Maturity Securities (Investments in Fixed Maturity Securities) (Details) (USD $)	Mar. 31, 2013		Dec. 31, 2012
Total held to maturity, Amortized Cost	$ 102,303,000		$ 102,951,000
Total held to maturity, Gross Unrealized Gains	23,207,000		24,420,000
Total held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	125,510,000		127,371,000
Total held to maturity, Carrying Value	102,303,000		102,951,000
Total available for sale, Amortized Cost	10,851,289,000		11,115,003,000
Total available for sale, Gross Unrealized Gains	738,817,000		758,662,000
Total available for sale, Gross Unrealized Losses	(27,292,000)		(32,660,000)
Total available for sale, Fair Value	11,562,814,000		11,841,005,000
Total available for sale, Carrying Value	11,562,814,000		11,841,005,000
Total investment in fixed maturity securities, Amortized Cost	10,953,592,000		11,217,954,000
Total investments in fixed maturity securities, Gross Unrealized Gains	762,024,000		783,082,000
Total investments in fixed maturity securities, Gross Unrealized Losses	(27,292,000)		(32,660,000)
Total investments in fixed maturity securities, Fair Value	11,688,324,000		11,968,376,000
Total investments in fixed maturity securities, carrying value	11,665,117,000		11,943,956,000
Minimum ownership of Common stockholders equity	10.00%
U.S. Government And Government Agency [Member]
Total available for sale, Amortized Cost	807,341,000		827,591,000
Total available for sale, Gross Unrealized Gains	68,429,000		72,532,000
Total available for sale, Gross Unrealized Losses	(58,000)		(1,660,000)
Total available for sale, Fair Value	875,712,000		898,463,000
Total available for sale, Carrying Value	875,712,000		898,463,000
State And Municipal [Member]
Total available for sale, Amortized Cost	4,110,714,000		4,449,238,000
Total available for sale, Gross Unrealized Gains	308,121,000		328,974,000
Total available for sale, Gross Unrealized Losses	(9,219,000)		(9,693,000)
Total available for sale, Fair Value	4,409,616,000		4,768,519,000
Total available for sale, Carrying Value	4,409,616,000		4,768,519,000
Residential Mortgage-Backed [Member]
Total available for sale, Amortized Cost	1,293,875,000	[1]	1,395,739,000	[1]
Total available for sale, Gross Unrealized Gains	48,550,000	[1]	53,846,000	[1]
Total available for sale, Gross Unrealized Losses	(6,878,000)	[1]	(7,456,000)	[1]
Total available for sale, Fair Value	1,335,547,000	[1]	1,442,129,000	[1]
Total available for sale, Carrying Value	1,335,547,000	[1]	1,442,129,000	[1]
Non-credit portion of other-than-temporary impairments ("OTTI")	1,925,000		3,037,000
Commercial Mortgage-Backed Securities [Member]
Total available for sale, Amortized Cost	203,260,000		268,671,000
Total available for sale, Gross Unrealized Gains	6,504,000		5,641,000
Total available for sale, Gross Unrealized Losses	(398,000)		(744,000)
Total available for sale, Fair Value	209,366,000		273,568,000
Total available for sale, Carrying Value	209,366,000		273,568,000
Corporate [Member]
Total available for sale, Amortized Cost	3,370,627,000		3,144,480,000
Total available for sale, Gross Unrealized Gains	210,832,000		214,322,000
Total available for sale, Gross Unrealized Losses	(10,380,000)		(12,083,000)
Total available for sale, Fair Value	3,571,079,000		3,346,719,000
Total available for sale, Carrying Value	3,571,079,000		3,346,719,000
Foreign [Member]
Total available for sale, Amortized Cost	1,065,472,000		1,029,284,000
Total available for sale, Gross Unrealized Gains	96,381,000		83,347,000
Total available for sale, Gross Unrealized Losses	(359,000)		(1,024,000)
Total available for sale, Fair Value	1,161,494,000		1,111,607,000
Total available for sale, Carrying Value	1,161,494,000		1,111,607,000
State And Municipal [Member]
Total held to maturity, Amortized Cost	66,125,000		65,190,000
Total held to maturity, Gross Unrealized Gains	17,675,000		18,529,000
Total held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	83,800,000		83,719,000
Total held to maturity, Carrying Value	66,125,000		65,190,000
Residential Mortgage-Backed [Member]
Total held to maturity, Amortized Cost	31,181,000		32,764,000
Total held to maturity, Gross Unrealized Gains	4,977,000		5,286,000
Total held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	36,158,000		38,050,000
Total held to maturity, Carrying Value	31,181,000		32,764,000
Corporate [Member]
Total held to maturity, Amortized Cost	4,997,000		4,997,000
Total held to maturity, Gross Unrealized Gains	555,000		605,000
Total held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	5,552,000		5,602,000
Total held to maturity, Carrying Value	$ 4,997,000		$ 4,997,000

[1] Gross unrealized losses for residential mortgage-backed securities include $1,925,000 and $3,037,000 as of March 31, 2013 and December 31, 2012, respectively, related to the non-credit portion of other-than-temporary impairments (“OTTI”) recognized in other comprehensive income.