Investments in Fixed Maturity Securities (Investments in Fixed Maturity Securities) (Details) (USD $)	Sep. 30, 2013		Dec. 31, 2012
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Total held to maturity, Amortized Cost	$ 101,550,000		$ 102,951,000
Held to Maturity Securities Accumulated Unrecognized Holding Gain	16,803,000		24,420,000
Held to Maturity Securities Accumulated Unrecognized Holding Loss	0		0
Total held to maturity, Fair Value	118,353,000		127,371,000
Total held to maturity, Carrying Value	101,550,000		102,951,000
Total available for sale, Amortized Cost	11,123,058,000		11,115,003,000
Total available for sale, Gross Unrealized Gains	477,539,000		758,662,000
Total available for sale, Gross Unrealized Losses	(90,100,000)		(32,660,000)
Total available for sale, Fair Value	11,510,497,000		11,841,005,000
Total available for sale, Carrying Value	11,510,497,000		11,841,005,000
Total investment in fixed maturity securities, Amortized Cost	11,224,608,000		11,217,954,000
Total investments in fixed maturity securities, Gross Unrealized Gains	494,342,000		783,082,000
Total investments in fixed maturity securities, Gross Unrealized Losses	(90,100,000)		(32,660,000)
Total investments in fixed maturity securities, Fair Value	11,628,850,000		11,968,376,000
Total investments in fixed maturity securities, carrying value	11,612,047,000		11,943,956,000
Minimum ownership of Common stockholders equity	10.00%
State and municipal
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Total available for sale, Amortized Cost	3,954,226,000		4,449,238,000
Total available for sale, Gross Unrealized Gains	185,876,000		328,974,000
Total available for sale, Gross Unrealized Losses	(21,965,000)		(9,693,000)
Total available for sale, Fair Value	4,118,137,000		4,768,519,000
Total available for sale, Carrying Value	4,118,137,000		4,768,519,000
U.S. government and agency
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Total available for sale, Amortized Cost	787,326,000		827,591,000
Total available for sale, Gross Unrealized Gains	42,195,000		72,532,000
Total available for sale, Gross Unrealized Losses	(4,462,000)		(1,660,000)
Total available for sale, Fair Value	825,059,000		898,463,000
Total available for sale, Carrying Value	825,059,000		898,463,000
Residential Mortgage-Backed [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Total available for sale, Amortized Cost	1,350,961,000	[1]	1,395,739,000	[1]
Total available for sale, Gross Unrealized Gains	33,231,000	[1]	53,846,000	[1]
Total available for sale, Gross Unrealized Losses	(21,653,000)	[1]	(7,456,000)	[1]
Total available for sale, Fair Value	1,362,539,000	[1]	1,442,129,000	[1]
Total available for sale, Carrying Value	1,362,539,000	[1]	1,442,129,000	[1]
Non-credit portion of other-than-temporary impairments ("OTTI")	2,216,000		3,037,000
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Total available for sale, Amortized Cost	126,036,000		268,671,000
Total available for sale, Gross Unrealized Gains	6,135,000		5,641,000
Total available for sale, Gross Unrealized Losses	(1,245,000)		(744,000)
Total available for sale, Fair Value	130,926,000		273,568,000
Total available for sale, Carrying Value	130,926,000		273,568,000
Corporate
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Total available for sale, Amortized Cost	3,800,276,000		3,144,480,000
Total available for sale, Gross Unrealized Gains	128,909,000		214,322,000
Total available for sale, Gross Unrealized Losses	(28,713,000)		(12,083,000)
Total available for sale, Fair Value	3,900,472,000		3,346,719,000
Total available for sale, Carrying Value	3,900,472,000		3,346,719,000
Foreign
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Total available for sale, Amortized Cost	1,104,233,000		1,029,284,000
Total available for sale, Gross Unrealized Gains	81,193,000		83,347,000
Total available for sale, Gross Unrealized Losses	(12,062,000)		(1,024,000)
Total available for sale, Fair Value	1,173,364,000		1,111,607,000
Total available for sale, Carrying Value	1,173,364,000		1,111,607,000
State and municipal
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Total held to maturity, Amortized Cost	67,982,000		65,190,000
Held to Maturity Securities Accumulated Unrecognized Holding Gain	12,488,000		18,529,000
Held to Maturity Securities Accumulated Unrecognized Holding Loss	0		0
Total held to maturity, Fair Value	80,470,000		83,719,000
Total held to maturity, Carrying Value	67,982,000		65,190,000
Residential Mortgage-Backed [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Total held to maturity, Amortized Cost	28,570,000		32,764,000
Held to Maturity Securities Accumulated Unrecognized Holding Gain	3,845,000		5,286,000
Held to Maturity Securities Accumulated Unrecognized Holding Loss	0		0
Total held to maturity, Fair Value	32,415,000		38,050,000
Total held to maturity, Carrying Value	28,570,000		32,764,000
Corporate
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
Total held to maturity, Amortized Cost	4,998,000		4,997,000
Held to Maturity Securities Accumulated Unrecognized Holding Gain	470,000		605,000
Held to Maturity Securities Accumulated Unrecognized Holding Loss	0		0
Total held to maturity, Fair Value	5,468,000		5,602,000
Total held to maturity, Carrying Value	$ 4,998,000		$ 4,997,000

[1] Gross unrealized losses for residential mortgage-backed securities include $2,216,000 and $3,037,000 as of September 30, 2013 and December 31, 2012, respectively, related to the non-credit portion of other-than-temporary impairments (“OTTI”) recognized in accumulated other comprehensive income.