Investments (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	$ 5,856,432	$ 5,385,432
Gross unrealized gain	429,899	346,301
Gross unrealized loss	(4,550)	(12,899)
Fixed maturity securities - available for sale	6,281,781	5,718,834
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	496,746	296,303
Less than 12 months, Unrealized losses	(9,371)	(11,651)
12 months or more, Fair value	2,616	33,282
12 months or more, Unrealized losses	(135)	(1,248)
Total, Fair value	499,362	329,585
Total, Unrealized losses	(9,506)	(12,899)
Available for sale equity securities
Available-for-sale Equity Securities, Amortized Cost Basis	275,827	0
Gross unrealized gain	13,768
Gross unrealized loss	(4,956)
Available-for-sale Securities, Equity Securities	284,639	0
US Government Agencies Debt Securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	195,049	285,166
Gross unrealized gain	4,560	10,523
Gross unrealized loss	(2)	(10)
Fixed maturity securities - available for sale	199,607	295,679
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	13,984
Less than 12 months, Unrealized losses	(2)	(10)
12 months or more, Fair value	0	0
12 months or more, Unrealized losses	0	0
Total, Fair value	55,034	13,984
Total, Unrealized losses	(2)	(10)
US States and Political Subdivisions Debt Securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	969,966	999,940
Gross unrealized gain	96,027	85,528
Gross unrealized loss	(182)	(127)
Fixed maturity securities - available for sale	1,065,811	1,085,341
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	14,162	10,256
Less than 12 months, Unrealized losses	(182)	(107)
12 months or more, Fair value	0	899
12 months or more, Unrealized losses	0	(20)
Total, Fair value	14,162	11,155
Total, Unrealized losses	(182)	(127)
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	2,033,947	1,741,297
Gross unrealized gain	168,772	122,746
Gross unrealized loss	(2,388)	(155)
Fixed maturity securities - available for sale	2,200,331	1,863,888
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	155,902	21,856
Less than 12 months, Unrealized losses	(2,388)	(67)
12 months or more, Fair value	0	6,796
12 months or more, Unrealized losses	0	(88)
Total, Fair value	155,902	28,652
Total, Unrealized losses	(2,388)	(155)
Corporate Debt Securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	1,247,282	817,886
Gross unrealized gain	69,243	35,221
Gross unrealized loss	(1,355)	(6,774)
Fixed maturity securities - available for sale	1,315,170	846,333
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	85,245	154,856
Less than 12 months, Unrealized losses	(1,220)	(6,391)
12 months or more, Fair value	2,616	18,005
12 months or more, Unrealized losses	(135)	(383)
Total, Fair value	87,861	172,861
Total, Unrealized losses	(1,355)	(6,774)
Residential Mortgage Backed Securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	632,665	1,036,436
Gross unrealized gain	32,560	65,771
Gross unrealized loss	(338)	(2,121)
Fixed maturity securities - available for sale	664,887	1,100,086
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	49,486	32,430
Less than 12 months, Unrealized losses	(338)	(1,364)
12 months or more, Fair value	0	7,582
12 months or more, Unrealized losses	0	(757)
Total, Fair value	49,486	40,012
Total, Unrealized losses	(338)	(2,121)
Commercial Mortgage Backed Securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	482,808	244,535
Gross unrealized gain	41,748	15,162
Gross unrealized loss	(267)	(3,573)
Fixed maturity securities - available for sale	524,289	256,124
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	26,263	39,075
Less than 12 months, Unrealized losses	(267)	(3,573)
12 months or more, Fair value	0	0
12 months or more, Unrealized losses	0	0
Total, Fair value	26,263	39,075
Total, Unrealized losses	(267)	(3,573)
Asset-backed Securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	32,801	34,655
Gross unrealized gain	474	147
Gross unrealized loss	0	(56)
Fixed maturity securities - available for sale	33,275	34,746
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	19,648
Less than 12 months, Unrealized losses	0	(56)
12 months or more, Fair value	0	0
12 months or more, Unrealized losses	0	0
Total, Fair value	0	19,648
Total, Unrealized losses		(56)
Foreign Government Debt Securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	261,914	225,517
Gross unrealized gain	16,515	11,203
Gross unrealized loss	(18)	(83)
Fixed maturity securities - available for sale	278,411	236,637
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	7,007	4,198
Less than 12 months, Unrealized losses	(18)	(83)
12 months or more, Fair value	0	0
12 months or more, Unrealized losses	0	0
Total, Fair value	7,007	4,198
Total, Unrealized losses	(18)	(83)
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	103,647
Less than 12 months, Unrealized losses	(4,956)
12 months or more, Fair value	0
12 months or more, Unrealized losses	0
Total, Fair value	103,647
Total, Unrealized losses	$ (4,956)