Investments (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	$ 5,819,392	$ 5,856,432
Gross unrealized gain	380,124	429,899
Gross unrealized loss	(14,060)	(4,550)
Fixed maturity securities - available for sale	6,185,456	6,281,781
Gross unrealized losses and fair value of all available for sale fixed income securities that were in a continuous loss position
Less than 12 months, Fair value	836,914	496,746
Less than 12 months, Unrealized losses	(16,961)	(9,371)
12 months or more, Fair value	14,465	2,616
12 months or more, Unrealized losses	(878)	(135)
Total, Fair value	851,379	499,362
Total, Unrealized losses	(17,839)	(9,506)
Available for sale equity securities
Available-for-sale Equity Securities, Amortized Cost Basis	319,499	275,827
Gross unrealized gain	34,632	13,768
Gross unrealized loss	(3,779)	(4,956)
Available-for-sale Securities, Equity Securities	350,352	284,639
US Government Agencies Debt Securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	122,242	195,049
Gross unrealized gain	4,007	4,560
Gross unrealized loss	0	(2)
Fixed maturity securities - available for sale	126,249	199,607
Gross unrealized losses and fair value of all available for sale fixed income securities that were in a continuous loss position
Less than 12 months, Fair value		55,034
Less than 12 months, Unrealized losses		(2)
12 months or more, Fair value		0
12 months or more, Unrealized losses		0
Total, Fair value		55,034
Total, Unrealized losses		(2)
US States and Political Subdivisions Debt Securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	955,228	969,966
Gross unrealized gain	88,266	96,027
Gross unrealized loss	(493)	(182)
Fixed maturity securities - available for sale	1,043,001	1,065,811
Gross unrealized losses and fair value of all available for sale fixed income securities that were in a continuous loss position
Less than 12 months, Fair value	35,621	14,162
Less than 12 months, Unrealized losses	(493)	(182)
12 months or more, Fair value	0	0
12 months or more, Unrealized losses	0	0
Total, Fair value	35,621	14,162
Total, Unrealized losses	(493)	(182)
Special Purpose Revenue Bonds Of States Municipalities And Political Subdivisions [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	2,101,195	2,033,947
Gross unrealized gain	155,078	168,772
Gross unrealized loss	(5,083)	(2,388)
Fixed maturity securities - available for sale	2,251,190	2,200,331
Gross unrealized losses and fair value of all available for sale fixed income securities that were in a continuous loss position
Less than 12 months, Fair value	264,246	155,902
Less than 12 months, Unrealized losses	(5,067)	(2,388)
12 months or more, Fair value	1,970	0
12 months or more, Unrealized losses	(16)	0
Total, Fair value	266,216	155,902
Total, Unrealized losses	(5,083)	(2,388)
Corporate Debt Securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	1,248,289	1,247,282
Gross unrealized gain	58,310	69,243
Gross unrealized loss	(4,974)	(1,355)
Fixed maturity securities - available for sale	1,301,625	1,315,170
Gross unrealized losses and fair value of all available for sale fixed income securities that were in a continuous loss position
Less than 12 months, Fair value	211,763	85,245
Less than 12 months, Unrealized losses	(4,112)	(1,220)
12 months or more, Fair value	12,495	2,616
12 months or more, Unrealized losses	(862)	(135)
Total, Fair value	224,258	87,861
Total, Unrealized losses	(4,974)	(1,355)
Residential Mortgage Backed Securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	630,822	632,665
Gross unrealized gain	26,807	32,560
Gross unrealized loss	(2,006)	(338)
Fixed maturity securities - available for sale	655,623	664,887
Gross unrealized losses and fair value of all available for sale fixed income securities that were in a continuous loss position
Less than 12 months, Fair value	136,267	49,486
Less than 12 months, Unrealized losses	(2,006)	(338)
12 months or more, Fair value	0	0
12 months or more, Unrealized losses	0	0
Total, Fair value	136,267	49,486
Total, Unrealized losses	(2,006)	(338)
Commercial Mortgage Backed Securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	488,000	482,808
Gross unrealized gain	35,107	41,748
Gross unrealized loss	(1,044)	(267)
Fixed maturity securities - available for sale	522,063	524,289
Gross unrealized losses and fair value of all available for sale fixed income securities that were in a continuous loss position
Less than 12 months, Fair value	83,541	26,263
Less than 12 months, Unrealized losses	(1,044)	(267)
12 months or more, Fair value	0	0
12 months or more, Unrealized losses	0	0
Total, Fair value	83,541	26,263
Total, Unrealized losses	(1,044)	(267)
Asset-backed Securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	42,015	32,801
Gross unrealized gain	515	474
Gross unrealized loss	0	0
Fixed maturity securities - available for sale	42,530	33,275
Gross unrealized losses and fair value of all available for sale fixed income securities that were in a continuous loss position
Less than 12 months, Fair value	0
Less than 12 months, Unrealized losses	0
12 months or more, Fair value	0
12 months or more, Unrealized losses	0
Total, Fair value	0
Total, Unrealized losses	0
Foreign Government Debt Securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	231,601	261,914
Gross unrealized gain	12,034	16,515
Gross unrealized loss	(460)	(18)
Fixed maturity securities - available for sale	243,175	278,411
Gross unrealized losses and fair value of all available for sale fixed income securities that were in a continuous loss position
Less than 12 months, Fair value	46,250	7,007
Less than 12 months, Unrealized losses	(460)	(18)
12 months or more, Fair value	0	0
12 months or more, Unrealized losses	0	0
Total, Fair value	46,250	7,007
Total, Unrealized losses	(460)	(18)
Equity Securities [Member]
Gross unrealized losses and fair value of all available for sale fixed income securities that were in a continuous loss position
Less than 12 months, Fair value	59,226	103,647
Less than 12 months, Unrealized losses	(3,779)	(4,956)
12 months or more, Fair value	0	0
12 months or more, Unrealized losses	0	0
Total, Fair value	59,226	103,647
Total, Unrealized losses	$ (3,779)	$ (4,956)