Investments (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	$ 5,847,178	$ 5,385,432
Gross unrealized gain	470,656	346,301
Gross unrealized loss	(2,469)	(12,899)
Fixed maturity securities - available for sale	6,315,365	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	195,562	296,303
Less than 12 months, Unrealized losses	(5,249)	(11,651)
12 months or more, Fair value	9,074	33,282
12 months or more, Unrealized losses	(566)	(1,248)
Total, Fair value	204,636	329,585
Available for sale equity securities
Available-for-sale Equity Securities, Amortized Cost Basis	197,469	0
Gross unrealized gain	8,741
Gross unrealized loss	(3,346)
Available-for-sale Securities, Equity Securities	202,864	0
US Government Agencies Debt Securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	224,267	285,166
Gross unrealized gain	7,769	10,523
Gross unrealized loss	0	(10)
Fixed maturity securities - available for sale	232,036	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	0	13,984
Less than 12 months, Unrealized losses	0	(10)
12 months or more, Fair value	0
12 months or more, Unrealized losses	0
Total, Fair value	0	13,984
US States and Political Subdivisions Debt Securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	977,682	999,940
Gross unrealized gain	101,493	85,528
Gross unrealized loss	0	(127)
Fixed maturity securities - available for sale	1,079,175	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	0	10,256
Less than 12 months, Unrealized losses	0	(107)
12 months or more, Fair value	0	899
12 months or more, Unrealized losses	0	(20)
Total, Fair value	0	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	1,895,991	1,741,297
Gross unrealized gain	180,049	122,746
Gross unrealized loss	(23)	(155)
Fixed maturity securities - available for sale	2,076,017	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	2,813	21,856
Less than 12 months, Unrealized losses	(23)	(67)
12 months or more, Fair value	0	6,796
12 months or more, Unrealized losses	0	(88)
Total, Fair value	2,813	28,652
Corporate Debt Securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	1,152,593	817,886
Gross unrealized gain	66,475	35,221
Gross unrealized loss	(1,761)	(6,774)
Fixed maturity securities - available for sale	1,217,307	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	89,993	154,856
Less than 12 months, Unrealized losses	(1,727)	(6,391)
12 months or more, Fair value	2,441	18,005
12 months or more, Unrealized losses	(34)	(383)
Total, Fair value	92,434	172,861
Residential Mortgage Backed Securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	789,172	1,036,436
Gross unrealized gain	52,999	65,771
Gross unrealized loss	(629)	(2,121)
Fixed maturity securities - available for sale	841,542	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	11,540	32,430
Less than 12 months, Unrealized losses	(97)	(1,364)
12 months or more, Fair value	6,633	7,582
12 months or more, Unrealized losses	(532)	(757)
Total, Fair value	18,173	40,012
Commercial Mortgage Backed Securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	479,517	244,535
Gross unrealized gain	44,635	15,162
Gross unrealized loss	(52)	(3,573)
Fixed maturity securities - available for sale	524,100	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	7,906	39,075
Less than 12 months, Unrealized losses	(52)	(3,573)
12 months or more, Fair value	0
12 months or more, Unrealized losses	0
Total, Fair value	7,906	39,075
Asset-backed Securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	47,354	34,655
Gross unrealized gain	370	147
Gross unrealized loss	0	(56)
Fixed maturity securities - available for sale	47,724	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
12 months or more, Unrealized losses	0
Total, Fair value	0	19,648
Foreign Government Debt Securities [Member]
Fixed Maturity & Equity securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	280,602	225,517
Gross unrealized gain	16,866	11,203
Gross unrealized loss	(4)	(83)
Fixed maturity securities - available for sale	297,464	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	9,259	4,198
Less than 12 months, Unrealized losses	(4)	(83)
12 months or more, Fair value	0
12 months or more, Unrealized losses	0
Total, Fair value	9,259	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	74,051
Less than 12 months, Unrealized losses	(3,346)
12 months or more, Fair value	0
12 months or more, Unrealized losses	0
Total, Fair value	$ 74,051