Investments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Fixed Maturity Securities, Amortized Cost Basis	$ 5,943,160	$ 5,856,432
Gross unrealized gain	235,082	429,899
Gross unrealized loss	(83,602)	(4,550)
Fixed maturity securities - available for sale	6,094,640	6,281,781
Gross unrealized losses and fair value of all available for sale fixed maturity securities that were in a continuous loss position
Less than 12 months, Fair value	1,872,370	496,746
Less than 12 months, Unrealized losses	(89,030)	(9,371)
12 months or more, Fair value	16,563	2,616
12 months or more, Unrealized losses	(1,150)	(135)
Total, Fair value	1,888,933	499,362
Total, Unrealized losses	(90,180)	(9,506)
Available for sale equity securities
Available-for-sale Equity Securities, Amortized Cost Basis	323,249	275,827
Gross unrealized gain	31,169	13,768
Gross unrealized loss	(6,578)	(4,956)
Available-for-sale Securities, Equity Securities	347,840	284,639
US Government and Government Agency Securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Fixed Maturity Securities, Amortized Cost Basis	120,951	195,049
Gross unrealized gain	3,076	4,560
Gross unrealized loss	(363)	(2)
Fixed maturity securities - available for sale	123,664	199,607
Gross unrealized losses and fair value of all available for sale fixed maturity securities that were in a continuous loss position
Less than 12 months, Fair value	20,667	55,034
Less than 12 months, Unrealized losses	(363)	(2)
12 months or more, Fair value	0	0
12 months or more, Unrealized losses	0	0
Total, Fair value	20,667	55,034
Total, Unrealized losses	(363)	(2)
Fixed Maturity Securities of States, Municipalities and Political Subdivisions Securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Fixed Maturity Securities, Amortized Cost Basis	974,153	969,966
Gross unrealized gain	60,281	96,027
Gross unrealized loss	(4,617)	(182)
Fixed maturity securities - available for sale	1,029,817	1,065,811
Gross unrealized losses and fair value of all available for sale fixed maturity securities that were in a continuous loss position
Less than 12 months, Fair value	97,408	14,162
Less than 12 months, Unrealized losses	(4,617)	(182)
12 months or more, Fair value	0	0
12 months or more, Unrealized losses	0	0
Total, Fair value	97,408	14,162
Total, Unrealized losses	(4,617)	(182)
Special Purpose Revenue Bonds Of States Municipalities And Political Subdivisions [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Fixed Maturity Securities, Amortized Cost Basis	2,218,302	2,033,947
Gross unrealized gain	91,093	168,772
Gross unrealized loss	(37,775)	(2,388)
Fixed maturity securities - available for sale	2,271,620	2,200,331
Gross unrealized losses and fair value of all available for sale fixed maturity securities that were in a continuous loss position
Less than 12 months, Fair value	648,672	155,902
Less than 12 months, Unrealized losses	(37,753)	(2,388)
12 months or more, Fair value	1,197	0
12 months or more, Unrealized losses	(22)	0
Total, Fair value	649,869	155,902
Total, Unrealized losses	(37,775)	(2,388)
Corporate Securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Fixed Maturity Securities, Amortized Cost Basis	1,215,761	1,247,282
Gross unrealized gain	34,077	69,243
Gross unrealized loss	(13,193)	(1,355)
Fixed maturity securities - available for sale	1,236,645	1,315,170
Gross unrealized losses and fair value of all available for sale fixed maturity securities that were in a continuous loss position
Less than 12 months, Fair value	419,283	85,245
Less than 12 months, Unrealized losses	(12,406)	(1,220)
12 months or more, Fair value	12,534	2,616
12 months or more, Unrealized losses	(787)	(135)
Total, Fair value	431,817	87,861
Total, Unrealized losses	(13,193)	(1,355)
Residential Mortgage Backed Securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Fixed Maturity Securities, Amortized Cost Basis	601,437	632,665
Gross unrealized gain	17,204	32,560
Gross unrealized loss	(13,146)	(338)
Fixed maturity securities - available for sale	605,495	664,887
Gross unrealized losses and fair value of all available for sale fixed maturity securities that were in a continuous loss position
Less than 12 months, Fair value	278,062	49,486
Less than 12 months, Unrealized losses	(13,146)	(338)
12 months or more, Fair value	0	0
12 months or more, Unrealized losses	0	0
Total, Fair value	278,062	49,486
Total, Unrealized losses	(13,146)	(338)
Commercial Mortgage Backed Securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Fixed Maturity Securities, Amortized Cost Basis	529,714	482,808
Gross unrealized gain	19,377	41,748
Gross unrealized loss	(12,892)	(267)
Fixed maturity securities - available for sale	536,199	524,289
Gross unrealized losses and fair value of all available for sale fixed maturity securities that were in a continuous loss position
Less than 12 months, Fair value	224,294	26,263
Less than 12 months, Unrealized losses	(12,892)	(267)
12 months or more, Fair value	0	0
12 months or more, Unrealized losses	0	0
Total, Fair value	224,294	26,263
Total, Unrealized losses	(12,892)	(267)
Asset-backed Securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Fixed Maturity Securities, Amortized Cost Basis	54,478	32,801
Gross unrealized gain	219	474
Gross unrealized loss	(406)	0
Fixed maturity securities - available for sale	54,291	33,275
Gross unrealized losses and fair value of all available for sale fixed maturity securities that were in a continuous loss position
Less than 12 months, Fair value	18,066
Less than 12 months, Unrealized losses	(406)
12 months or more, Fair value	0
12 months or more, Unrealized losses	0
Total, Fair value	18,066
Total, Unrealized losses	(406)
Foreign Government Securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Fixed Maturity Securities, Amortized Cost Basis	228,364	261,914
Gross unrealized gain	9,755	16,515
Gross unrealized loss	(1,210)	(18)
Fixed maturity securities - available for sale	236,909	278,411
Gross unrealized losses and fair value of all available for sale fixed maturity securities that were in a continuous loss position
Less than 12 months, Fair value	67,832	7,007
Less than 12 months, Unrealized losses	(1,210)	(18)
12 months or more, Fair value	0	0
12 months or more, Unrealized losses	0	0
Total, Fair value	67,832	7,007
Total, Unrealized losses	(1,210)	(18)
Equity Securities [Member]
Gross unrealized losses and fair value of all available for sale fixed maturity securities that were in a continuous loss position
Less than 12 months, Fair value	98,086	103,647
Less than 12 months, Unrealized losses	(6,237)	(4,956)
12 months or more, Fair value	2,832	0
12 months or more, Unrealized losses	(341)	0
Total, Fair value	100,918	103,647
Total, Unrealized losses	$ (6,578)	$ (4,956)