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Investments (Details) (USD $)
In Thousands
Sep. 30, 2011
Dec. 31, 2010
Fixed Income securities Available for Sale  
Available-for-sale Debt Securities, Amortized Cost Basis$ 5,356,333$ 4,864,806
Gross unrealized gain311,599170,204
Gross unrealized loss(16,743)(35,570)
Fixed income securities - available for sale5,651,1894,999,440
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 value453,1601,148,078
Less than 12 months, Unrealized losses(15,041)(34,151)
12 months or more, Fair value55,54533,859
12 months or more, Unrealized losses(1,702)(1,419)
Total, Fair value508,7051,181,937
U.S. government and government agency securities [Member]
  
Fixed Income securities Available for Sale  
Available-for-sale Debt Securities, Amortized Cost Basis281,758315,339
Gross unrealized gain11,5879,097
Gross unrealized loss(1)(169)
Fixed income securities - available for sale293,344324,267
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 value2,25020,976
Less than 12 months, Unrealized losses(1)(169)
12 months or more, Fair value00
12 months or more, Unrealized losses00
Total, Fair value2,25020,976
Fixed income securities of states, municipalities and political subdivisions [Member]
  
Fixed Income securities Available for Sale  
Available-for-sale Debt Securities, Amortized Cost Basis1,005,8371,050,969
Gross unrealized gain74,21038,825
Gross unrealized loss(188)(7,737)
Fixed income securities - available for sale1,079,8591,082,057
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 value15,360228,228
Less than 12 months, Unrealized losses(119)(7,621)
12 months or more, Fair value1,4762,279
12 months or more, Unrealized losses(69)(116)
Total, Fair value16,836230,507
Special purpose revenue bonds of states, municipalities and political subdivisions [Member]
  
Fixed Income securities Available for Sale  
Available-for-sale Debt Securities, Amortized Cost Basis1,717,3071,614,554
Gross unrealized gain99,27934,764
Gross unrealized loss(627)(21,259)
Fixed income securities - available for sale1,815,9591,628,059
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 value73,022689,190
Less than 12 months, Unrealized losses(441)(21,156)
12 months or more, Fair value29,0746,344
12 months or more, Unrealized losses(186)(103)
Total, Fair value102,096695,534
Corporate fixed income securities [Member]
  
Fixed Income securities Available for Sale  
Available-for-sale Debt Securities, Amortized Cost Basis785,733545,883
Gross unrealized gain34,63526,436
Gross unrealized loss(6,762)(1,925)
Fixed income securities - available for sale813,606570,394
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 value192,72766,029
Less than 12 months, Unrealized losses(6,322)(1,925)
12 months or more, Fair value16,9640
12 months or more, Unrealized losses(440)0
Total, Fair value209,69166,029
Residential mortgage-backed securities [Member]
  
Fixed Income securities Available for Sale  
Available-for-sale Debt Securities, Amortized Cost Basis1,045,247958,404
Gross unrealized gain69,26140,949
Gross unrealized loss(3,187)(4,245)
Fixed income securities - available for sale1,111,321995,108
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 value50,867123,782
Less than 12 months, Unrealized losses(2,180)(3,081)
12 months or more, Fair value8,03122,152
12 months or more, Unrealized losses(1,007)(1,164)
Total, Fair value58,898145,934
Commercial mortgage-backed securities [Member]
  
Fixed Income securities Available for Sale  
Available-for-sale Debt Securities, Amortized Cost Basis245,651136,746
Gross unrealized gain9,1808,518
Gross unrealized loss(5,780)(36)
Fixed income securities - available for sale249,051145,228
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 value94,5160
Less than 12 months, Unrealized losses(5,780)0
12 months or more, Fair value03,084
12 months or more, Unrealized losses0(36)
Total, Fair value94,5163,084
Asset-backed securities [Member]
  
Fixed Income securities Available for Sale  
Available-for-sale Debt Securities, Amortized Cost Basis34,48912,563
Gross unrealized gain17978
Gross unrealized loss(51)(75)
Fixed income securities - available for sale34,61712,566
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 value17,8169,174
Less than 12 months, Unrealized losses(51)(75)
12 months or more, Fair value00
12 months or more, Unrealized losses00
Total, Fair value17,8169,174
Foreign government securities [Member]
  
Fixed Income securities Available for Sale  
Available-for-sale Debt Securities, Amortized Cost Basis240,311230,348
Gross unrealized gain13,26811,537
Gross unrealized loss(147)(124)
Fixed income securities - available for sale253,432241,761
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 value6,60210,699
Less than 12 months, Unrealized losses(147)(124)
12 months or more, Fair value00
12 months or more, Unrealized losses00
Total, Fair value$ 6,602$ 10,699