Investments (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Fixed Maturity Securities, Amortized Cost Basis	$ 6,059,853	$ 5,856,432
Gross unrealized gain	219,903	429,899
Gross unrealized loss	(85,131)	(4,550)
Fixed maturity securities - available for sale	6,194,625	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	1,889,606	496,746
Less than 12 months, Unrealized losses	(88,129)	(9,371)
12 months or more, Fair value	26,897	2,616
12 months or more, Unrealized losses	(1,698)	(135)
Total, Fair value	1,916,503	499,362
Total, Unrealized losses	(89,827)	(9,506)
Available for sale equity securities
Available-for-sale Equity Securities, Amortized Cost Basis	395,018	275,827
Gross unrealized gain	43,023	13,768
Gross unrealized loss	(4,696)	(4,956)
Available-for-sale Securities, Equity Securities	433,345	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	115,109	195,049
Gross unrealized gain	2,698	4,560
Gross unrealized loss	(302)	(2)
Fixed maturity securities - available for sale	117,505	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	20,730	55,034
Less than 12 months, Unrealized losses	(302)	(2)
12 months or more, Fair value	0	0
12 months or more, Unrealized losses	0	0
Total, Fair value	20,730	55,034
Total, Unrealized losses	(302)	(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	965,526	969,966
Gross unrealized gain	55,813	96,027
Gross unrealized loss	(5,200)	(182)
Fixed maturity securities - available for sale	1,016,139	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	134,217	14,162
Less than 12 months, Unrealized losses	(5,200)	(182)
12 months or more, Fair value	0	0
12 months or more, Unrealized losses	0	0
Total, Fair value	134,217	14,162
Total, Unrealized losses	(5,200)	(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	2,258,661	2,033,947
Gross unrealized gain	81,517	168,772
Gross unrealized loss	(43,340)	(2,388)
Fixed maturity securities - available for sale	2,296,838	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	724,983	155,902
Less than 12 months, Unrealized losses	(43,322)	(2,388)
12 months or more, Fair value	1,323	0
12 months or more, Unrealized losses	(18)	0
Total, Fair value	726,306	155,902
Total, Unrealized losses	(43,340)	(2,388)
Corporate Securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Fixed Maturity Securities, Amortized Cost Basis	1,267,387	1,247,282
Gross unrealized gain	40,314	69,243
Gross unrealized loss	(9,997)	(1,355)
Fixed maturity securities - available for sale	1,297,704	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	307,327	85,245
Less than 12 months, Unrealized losses	(9,331)	(1,220)
12 months or more, Fair value	14,821	2,616
12 months or more, Unrealized losses	(666)	(135)
Total, Fair value	322,148	87,861
Total, Unrealized losses	(9,997)	(1,355)
Residential Mortgage Backed Securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Fixed Maturity Securities, Amortized Cost Basis	588,696	632,665
Gross unrealized gain	17,671	32,560
Gross unrealized loss	(13,410)	(338)
Fixed maturity securities - available for sale	592,957	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	270,062	49,486
Less than 12 months, Unrealized losses	(13,410)	(338)
12 months or more, Fair value	0	0
12 months or more, Unrealized losses	0	0
Total, Fair value	270,062	49,486
Total, Unrealized losses	(13,410)	(338)
Commercial Mortgage Backed Securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Fixed Maturity Securities, Amortized Cost Basis	524,706	482,808
Gross unrealized gain	18,519	41,748
Gross unrealized loss	(12,009)	(267)
Fixed maturity securities - available for sale	531,216	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	212,589	26,263
Less than 12 months, Unrealized losses	(11,414)	(267)
12 months or more, Fair value	5,042	0
12 months or more, Unrealized losses	(595)	0
Total, Fair value	217,631	26,263
Total, Unrealized losses	(12,009)	(267)
Asset-backed Securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Fixed Maturity Securities, Amortized Cost Basis	141,594	32,801
Gross unrealized gain	282	474
Gross unrealized loss	(575)	0
Fixed maturity securities - available for sale	141,301	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	49,647
Less than 12 months, Unrealized losses	(575)
12 months or more, Fair value	0
12 months or more, Unrealized losses	0
Total, Fair value	49,647
Total, Unrealized losses	(575)
Foreign Government Securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Fixed Maturity Securities, Amortized Cost Basis	198,174	261,914
Gross unrealized gain	3,089	16,515
Gross unrealized loss	(298)	(18)
Fixed maturity securities - available for sale	200,965	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	74,354	7,007
Less than 12 months, Unrealized losses	(298)	(18)
12 months or more, Fair value	0	0
12 months or more, Unrealized losses	0	0
Total, Fair value	74,354	7,007
Total, Unrealized losses	(298)	(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	95,697	103,647
Less than 12 months, Unrealized losses	(4,277)	(4,956)
12 months or more, Fair value	5,711	0
12 months or more, Unrealized losses	(419)	0
Total, Fair value	101,408	103,647
Total, Unrealized losses	$ (4,696)	$ (4,956)