Investments (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Fixed Income securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	$ 5,385,432	$ 4,864,806
Gross unrealized gain	346,301	170,204
Gross unrealized loss	(12,899)	(35,570)
Fixed income securities - available for sale	5,718,834	4,999,440
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	296,303	1,148,078
Less than 12 months, Unrealized losses	(11,651)	(34,151)
12 months or more, Fair value	33,282	33,859
12 months or more, Unrealized losses	(1,248)	(1,419)
Total, Fair value	329,585	1,181,937
U.S. government and government agency securities [Member]
Fixed Income securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	285,166	315,339
Gross unrealized gain	10,523	9,097
Gross unrealized loss	(10)	(169)
Fixed income securities - available for sale	295,679	324,267
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	13,984	20,976
Less than 12 months, Unrealized losses	(10)	(169)
12 months or more, Fair value	0	0
12 months or more, Unrealized losses	0	0
Total, Fair value	13,984	20,976
Fixed income securities of states, municipalities and political subdivisions [Member]
Fixed Income securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	999,940	1,050,969
Gross unrealized gain	85,528	38,825
Gross unrealized loss	(127)	(7,737)
Fixed income securities - available for sale	1,085,341	1,082,057
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	10,256	228,228
Less than 12 months, Unrealized losses	(107)	(7,621)
12 months or more, Fair value	899	2,279
12 months or more, Unrealized losses	(20)	(116)
Total, Fair value	11,155	230,507
Special purpose revenue bonds of states, municipalities and political subdivisions [Member]
Fixed Income securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	1,741,297	1,614,554
Gross unrealized gain	122,746	34,764
Gross unrealized loss	(155)	(21,259)
Fixed income securities - available for sale	1,863,888	1,628,059
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	21,856	689,190
Less than 12 months, Unrealized losses	(67)	(21,156)
12 months or more, Fair value	6,796	6,344
12 months or more, Unrealized losses	(88)	(103)
Total, Fair value	28,652	695,534
Corporate fixed income securities [Member]
Fixed Income securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	817,886	545,883
Gross unrealized gain	35,221	26,436
Gross unrealized loss	(6,774)	(1,925)
Fixed income securities - available for sale	846,333	570,394
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	154,856	66,029
Less than 12 months, Unrealized losses	(6,391)	(1,925)
12 months or more, Fair value	18,005	0
12 months or more, Unrealized losses	(383)	0
Total, Fair value	172,861	66,029
Residential mortgage-backed securities [Member]
Fixed Income securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	1,036,436	958,404
Gross unrealized gain	65,771	40,949
Gross unrealized loss	(2,121)	(4,245)
Fixed income securities - available for sale	1,100,086	995,108
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	32,430	123,782
Less than 12 months, Unrealized losses	(1,364)	(3,081)
12 months or more, Fair value	7,582	22,152
12 months or more, Unrealized losses	(757)	(1,164)
Total, Fair value	40,012	145,934
Commercial mortgage-backed securities [Member]
Fixed Income securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	244,535	136,746
Gross unrealized gain	15,162	8,518
Gross unrealized loss	(3,573)	(36)
Fixed income securities - available for sale	256,124	145,228
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	39,075	0
Less than 12 months, Unrealized losses	(3,573)	0
12 months or more, Fair value	0	3,084
12 months or more, Unrealized losses	0	(36)
Total, Fair value	39,075	3,084
Asset-backed securities [Member]
Fixed Income securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	34,655	12,563
Gross unrealized gain	147	78
Gross unrealized loss	(56)	(75)
Fixed income securities - available for sale	34,746	12,566
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	19,648	9,174
Less than 12 months, Unrealized losses	(56)	(75)
12 months or more, Fair value	0	0
12 months or more, Unrealized losses	0	0
Total, Fair value	19,648	9,174
Foreign government securities [Member]
Fixed Income securities Available for Sale
Available-for-sale Debt Securities, Amortized Cost Basis	225,517	230,348
Gross unrealized gain	11,203	11,537
Gross unrealized loss	(83)	(124)
Fixed income securities - available for sale	236,637	241,761
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	4,198	10,699
Less than 12 months, Unrealized losses	(83)	(124)
12 months or more, Fair value	0	0
12 months or more, Unrealized losses	0	0
Total, Fair value	$ 4,198	$ 10,699