Investments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	$ 5,712,890	$ 5,385,432
Gross unrealized gain	392,459	346,301
Gross unrealized loss	(7,483)	(12,899)
Fixed maturity securities - available for sale	6,097,866	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	424,935	296,303
Less than 12 months, Unrealized losses	(7,933)	(11,651)
12 months or more, Fair value	9,035	33,282
12 months or more, Unrealized losses	(1,427)	(1,248)
Total, Fair value	433,970	329,585
Available for sale equity securities
Available-for-sale Equity Securities, Amortized Cost Basis	92,580	0
Gross unrealized gain	2,957
Gross unrealized loss	(1,877)
Available-for-sale Securities, Equity Securities	93,660	0
U.S. government and government agency securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	220,995	285,166
Gross unrealized gain	7,991	10,523
Gross unrealized loss	(15)	(10)
Fixed maturity securities - available for sale	228,971	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	34,791	13,984
Less than 12 months, Unrealized losses	(15)	(10)
12 months or more, Fair value	0
12 months or more, Unrealized losses	0
Total, Fair value	34,791	13,984
Fixed income securities of states, municipalities and political subdivisions [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	982,569	999,940
Gross unrealized gain	91,307	85,528
Gross unrealized loss	(8)	(127)
Fixed maturity securities - available for sale	1,073,868	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	2,177	10,256
Less than 12 months, Unrealized losses	(8)	(107)
12 months or more, Fair value	0	899
12 months or more, Unrealized losses	0	(20)
Total, Fair value	2,177	11,155
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	1,858,848	1,741,297
Gross unrealized gain	148,858	122,746
Gross unrealized loss	(158)	(155)
Fixed maturity securities - available for sale	2,007,548	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	36,514	21,856
Less than 12 months, Unrealized losses	(143)	(67)
12 months or more, Fair value	1,288	6,796
12 months or more, Unrealized losses	(15)	(88)
Total, Fair value	37,802	28,652
Corporate fixed income securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	1,088,342	817,886
Gross unrealized gain	46,363	35,221
Gross unrealized loss	(5,048)	(6,774)
Fixed maturity securities - available for sale	1,129,657	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	235,506	154,856
Less than 12 months, Unrealized losses	(5,048)	(6,391)
12 months or more, Fair value	0	18,005
12 months or more, Unrealized losses	0	(383)
Total, Fair value	235,506	172,861
Residential mortgage-backed securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	854,262	1,036,436
Gross unrealized gain	50,485	65,771
Gross unrealized loss	(1,915)	(2,121)
Fixed maturity securities - available for sale	902,832	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	18,221	32,430
Less than 12 months, Unrealized losses	(503)	(1,364)
12 months or more, Fair value	7,747	7,582
12 months or more, Unrealized losses	(1,412)	(757)
Total, Fair value	25,968	40,012
Commercial mortgage-backed securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	382,595	244,535
Gross unrealized gain	34,109	15,162
Gross unrealized loss	(4)	(3,573)
Fixed maturity securities - available for sale	416,700	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	10,844	39,075
Less than 12 months, Unrealized losses	(4)	(3,573)
12 months or more, Fair value	0
12 months or more, Unrealized losses	0
Total, Fair value	10,844	39,075
Asset-backed securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	45,954	34,655
Gross unrealized gain	237	147
Gross unrealized loss	(9)	(56)
Fixed maturity securities - available for sale	46,182	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	10,263	19,648
Less than 12 months, Unrealized losses	(9)	(56)
12 months or more, Fair value	0
12 months or more, Unrealized losses	0
Total, Fair value	10,263	19,648
Foreign government securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	279,325	225,517
Gross unrealized gain	13,109	11,203
Gross unrealized loss	(326)	(83)
Fixed maturity securities - available for sale	292,108	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	43,739	4,198
Less than 12 months, Unrealized losses	(326)	(83)
12 months or more, Fair value	0
12 months or more, Unrealized losses	0
Total, Fair value	43,739	4,198
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	32,880
Less than 12 months, Unrealized losses	(1,877)
12 months or more, Fair value	0
12 months or more, Unrealized losses	0
Total, Fair value	$ 32,880