Investments in Fixed Maturity Securities (Investments in Fixed Maturity Securities) (Details) (USD $)	Dec. 31, 2011		Dec. 31, 2010
Total held to maturity, Amortized Cost	$ 115,109,000		$ 115,995,000
Total held to maturity, Gross Unrealized Gains	18,873,000		7,292,000
Total held to maturity, Gross Unrealized Losses	0		(1,129,000)
Total held to maturity, Fair Value	133,982,000		122,158,000
Total held to maturity, Carrying Value	115,109,000		115,995,000
Total available for sale, Amortized Cost	10,639,047,000		10,737,198,000
Total available for sale, Gross Unrealized Gains	632,430,000		449,840,000
Total available for sale, Gross Unrealized Losses	(74,549,000)		(93,879,000)
Total available for sale, Fair Value	11,196,928,000		11,093,159,000
Total available for sale, Carrying Value	11,196,928,000		11,093,159,000
Total investment in fixed maturity securities, Amortized Cost	10,754,156,000		10,853,193,000
Total investments in fixed maturity securities, Gross Unrealized Gains	651,303,000		457,132,000
Total investments in fixed maturity securities, Gross Unrealized Losses	(74,549,000)		(95,008,000)
Total investments in fixed maturity securities, Fair Value	11,330,910,000		11,215,317,000
Total investments in fixed maturity securities, carrying value	11,312,037,000		11,209,154,000
Minimum ownership of Common stockholders equity	10.00%
U.S. Government And Government Agency [Member]
Total available for sale, Amortized Cost	906,924,000		1,289,669,000
Total available for sale, Gross Unrealized Gains	69,920,000		58,658,000
Total available for sale, Gross Unrealized Losses	(351,000)		(452,000)
Total available for sale, Fair Value	976,493,000		1,347,875,000
Total available for sale, Carrying Value	976,493,000		1,347,875,000
State and Local Jurisdiction [Member]
Total available for sale, Amortized Cost	5,031,275,000		5,302,513,000
Total available for sale, Gross Unrealized Gains	308,345,000		203,221,000
Total available for sale, Gross Unrealized Losses	(16,550,000)		(44,288,000)
Total available for sale, Fair Value	5,323,070,000		5,461,446,000
Total available for sale, Carrying Value	5,323,070,000		5,461,446,000
Residential Mortgage Backed Securities [Member]
Total available for sale, Amortized Cost	1,416,427,000	[1]	1,319,289,000	[1]
Total available for sale, Gross Unrealized Gains	75,635,000	[1]	52,165,000	[1]
Total available for sale, Gross Unrealized Losses	(15,894,000)	[1]	(13,278,000)	[1]
Total available for sale, Fair Value	1,476,168,000	[1]	1,358,176,000	[1]
Total available for sale, Carrying Value	1,476,168,000	[1]	1,358,176,000	[1]
Non-credit portion of other-than-temporary impairments ("OTTI")	7,668,000		4,064,000
Commercial Mortgage-Backed Securities [Member]
Total available for sale, Amortized Cost	105,383,000		57,057,000
Total available for sale, Gross Unrealized Gains	4,054,000		2,207,000
Total available for sale, Gross Unrealized Losses	(1,018,000)		(5,594,000)
Total available for sale, Fair Value	108,419,000		53,670,000
Total available for sale, Carrying Value	108,419,000		53,670,000
Corporate Debt Securities [Member]
Total available for sale, Amortized Cost	2,328,200,000		2,055,513,000
Total available for sale, Gross Unrealized Gains	132,311,000		93,789,000
Total available for sale, Gross Unrealized Losses	(36,087,000)		(29,379,000)
Total available for sale, Fair Value	2,424,424,000		2,119,923,000
Total available for sale, Carrying Value	2,424,424,000		2,119,923,000
Foreign [Member]
Total available for sale, Amortized Cost	850,838,000		713,157,000
Total available for sale, Gross Unrealized Gains	42,165,000		39,800,000
Total available for sale, Gross Unrealized Losses	(4,649,000)		(888,000)
Total available for sale, Fair Value	888,354,000		752,069,000
Total available for sale, Carrying Value	888,354,000		752,069,000
State and Local Jurisdiction [Member]
Total held to maturity, Amortized Cost	74,354,000		71,998,000
Total held to maturity, Gross Unrealized Gains	12,546,000		3,440,000
Total held to maturity, Gross Unrealized Losses	0		(1,129,000)
Total held to maturity, Fair Value	86,900,000		74,309,000
Total held to maturity, Carrying Value	74,354,000		71,998,000
Residential Mortgage Backed Securities [Member]
Total held to maturity, Amortized Cost	35,759,000		39,002,000
Total held to maturity, Gross Unrealized Gains	5,610,000		3,667,000
Total held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	41,369,000		42,669,000
Total held to maturity, Carrying Value	35,759,000		39,002,000
Corporate Debt Securities [Member]
Total held to maturity, Amortized Cost	4,996,000		4,995,000
Total held to maturity, Gross Unrealized Gains	717,000		185,000
Total held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	5,713,000		5,180,000
Total held to maturity, Carrying Value	$ 4,996,000		$ 4,995,000

[1] Gross unrealized losses for mortgage-backed securities include $7,668,000 and $4,064,000, as of December 31, 2011 and 2010, respectively, related to the non-credit portion of OTTI recognized in other comprehensive income.