Investments in Fixed Maturity Securities (Investments in Fixed Maturity Securities) (Details) (USD $)	Jun. 30, 2011		Dec. 31, 2010
Total held to maturity, Amortized Cost	$ 116,232,000		$ 115,995,000
Total held to maturity, Gross Unrealized Gains	12,577,000		7,292,000
Total held to maturity, Gross Unrealized Losses			(1,129,000)
Total held to maturity, Fair Value	128,809,000		122,158,000
Total held to maturity, Carrying Value	116,232,000		115,995,000
Total available for sale, Amortized Cost	10,733,893,000		10,737,198,000
Total available for sale, Gross Unrealized Gains	491,874,000		449,840,000
Total available for sale, Gross Unrealized Losses	(62,490,000)		(93,879,000)
Total available for sale, Fair Value	11,163,277,000		11,093,159,000
Total available for sale, Carrying Value	11,163,277,000		11,093,159,000
Total investments in fixed maturity securities, Amortized Cost	10,850,125,000		10,853,193,000
Total investments in fixed maturity securities, Gross Unrealized Gains	504,451,000		457,132,000
Total investments in fixed maturity securities, Gross Unrealized Losses	(62,490,000)		(95,008,000)
Total investments in fixed maturity securities, Fair Value	11,292,086,000		11,215,317,000
Total investments in fixed maturity securities, carrying value	11,279,509,000		11,209,154,000
Minimum ownership of Common stockholders equity	10.00%
State And Municipal [Member]
Total held to maturity, Amortized Cost	74,086,000		71,998,000
Total held to maturity, Gross Unrealized Gains	6,272,000		3,440,000
Total held to maturity, Gross Unrealized Losses			(1,129,000)
Total held to maturity, Fair Value	80,358,000		74,309,000
Total held to maturity, Carrying Value	74,086,000		71,998,000
State And Municipal [Member]
Total available for sale, Amortized Cost	5,178,823,000		5,302,513,000
Total available for sale, Gross Unrealized Gains	233,040,000		203,221,000
Total available for sale, Gross Unrealized Losses	(25,249,000)		(44,288,000)
Total available for sale, Fair Value	5,386,614,000		5,461,446,000
Total available for sale, Carrying Value	5,386,614,000		5,461,446,000
Residential Mortgage-Backed [Member]
Total held to maturity, Amortized Cost	37,151,000		39,002,000
Total held to maturity, Gross Unrealized Gains	5,577,000		3,667,000
Total held to maturity, Gross Unrealized Losses
Total held to maturity, Fair Value	42,728,000		42,669,000
Total held to maturity, Carrying Value	37,151,000		39,002,000
Residential Mortgage-Backed [Member]
Total available for sale, Amortized Cost	1,339,725,000	[1]	1,319,289,000	[1]
Total available for sale, Gross Unrealized Gains	56,022,000	[1]	52,165,000	[1]
Total available for sale, Gross Unrealized Losses	(12,258,000)	[1]	(13,278,000)	[1]
Total available for sale, Fair Value	1,383,489,000	[1]	1,358,176,000	[1]
Total available for sale, Carrying Value	1,383,489,000	[1]	1,358,176,000	[1]
Non-credit portion of other-than-temporary impairments ("OTTI")	4,720,000		4,064,000
Corporate [Member]
Total held to maturity, Amortized Cost	4,995,000		4,995,000
Total held to maturity, Gross Unrealized Gains	728,000		185,000
Total held to maturity, Gross Unrealized Losses
Total held to maturity, Fair Value	5,723,000		5,180,000
Total held to maturity, Carrying Value	4,995,000		4,995,000
Corporate [Member]
Total available for sale, Amortized Cost	2,524,002,000		2,307,987,000
Total available for sale, Gross Unrealized Gains	109,208,000		102,306,000
Total available for sale, Gross Unrealized Losses	(23,925,000)		(30,031,000)
Total available for sale, Fair Value	2,609,285,000		2,380,262,000
Total available for sale, Carrying Value	2,609,285,000		2,380,262,000
U.S. Government And Government Agency [Member]
Total available for sale, Amortized Cost	1,115,586,000		1,289,669,000
Total available for sale, Gross Unrealized Gains	62,294,000		58,658,000
Total available for sale, Gross Unrealized Losses	(591,000)		(452,000)
Total available for sale, Fair Value	1,177,289,000		1,347,875,000
Total available for sale, Carrying Value	1,177,289,000		1,347,875,000
Commercial Mortgage-Backed Securities [Member]
Total available for sale, Amortized Cost	54,076,000		57,057,000
Total available for sale, Gross Unrealized Gains	2,208,000		2,207,000
Total available for sale, Gross Unrealized Losses	(186,000)		(5,594,000)
Total available for sale, Fair Value	56,098,000		53,670,000
Total available for sale, Carrying Value	56,098,000		53,670,000
Foreign [Member]
Total available for sale, Amortized Cost	521,681,000		460,683,000
Total available for sale, Gross Unrealized Gains	29,102,000		31,283,000
Total available for sale, Gross Unrealized Losses	(281,000)		(236,000)
Total available for sale, Fair Value	550,502,000		491,730,000
Total available for sale, Carrying Value	$ 550,502,000		$ 491,730,000

[1] Gross unrealized losses for residential mortgage-backed securities include $4,720,000 and $4,064,000 as of June 30, 2011 and December 31, 2010, respectively, related to the non-credit portion of other-than-temporary impairments ("OTTI") recognized in other comprehensive income.