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Fair value measurements (Details) (USD $)
 3 Months Ended 3 Months Ended
Mar. 31, 2012
Dec. 31, 2011
Mar. 31, 2012
Obligations of U.S. States and Political Subdivisions [Member]
Mar. 31, 2011
Obligations of U.S. States and Political Subdivisions [Member]
Mar. 31, 2012
Corporate Debt Securities [Member]
Mar. 31, 2011
Corporate Debt Securities [Member]
Mar. 31, 2012
Equity Securities [Member]
Mar. 31, 2011
Equity Securities [Member]
Mar. 31, 2012
Total Investments [Member]
Mar. 31, 2011
Total Investments [Member]
Mar. 31, 2012
Real Estate Acquired [Member]
Mar. 31, 2011
Real Estate Acquired [Member]
Mar. 31, 2012
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Dec. 31, 2011
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Mar. 31, 2012
Significant Other Observable Inputs (Level 2) [Member]
Dec. 31, 2011
Significant Other Observable Inputs (Level 2) [Member]
Mar. 31, 2012
Significant Unobservable Inputs (Level 3) [Member]
Dec. 31, 2011
Significant Unobservable Inputs (Level 3) [Member]
Mar. 31, 2012
U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member]
Dec. 31, 2011
U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member]
Mar. 31, 2012
U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member]
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Dec. 31, 2011
U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member]
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Mar. 31, 2012
U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member]
Significant Other Observable Inputs (Level 2) [Member]
Dec. 31, 2011
U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member]
Significant Other Observable Inputs (Level 2) [Member]
Mar. 31, 2012
U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member]
Significant Unobservable Inputs (Level 3) [Member]
Dec. 31, 2011
U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member]
Significant Unobservable Inputs (Level 3) [Member]
Mar. 31, 2012
Obligations of U.S. States and Political Subdivisions [Member]
Dec. 31, 2011
Obligations of U.S. States and Political Subdivisions [Member]
Mar. 31, 2012
Obligations of U.S. States and Political Subdivisions [Member]
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Dec. 31, 2011
Obligations of U.S. States and Political Subdivisions [Member]
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Mar. 31, 2012
Obligations of U.S. States and Political Subdivisions [Member]
Significant Other Observable Inputs (Level 2) [Member]
Dec. 31, 2011
Obligations of U.S. States and Political Subdivisions [Member]
Significant Other Observable Inputs (Level 2) [Member]
Mar. 31, 2012
Obligations of U.S. States and Political Subdivisions [Member]
Significant Unobservable Inputs (Level 3) [Member]
Dec. 31, 2011
Obligations of U.S. States and Political Subdivisions [Member]
Significant Unobservable Inputs (Level 3) [Member]
Mar. 31, 2012
Corporate Debt Securities [Member]
Dec. 31, 2011
Corporate Debt Securities [Member]
Mar. 31, 2012
Corporate Debt Securities [Member]
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Dec. 31, 2011
Corporate Debt Securities [Member]
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Mar. 31, 2012
Corporate Debt Securities [Member]
Significant Other Observable Inputs (Level 2) [Member]
Dec. 31, 2011
Corporate Debt Securities [Member]
Significant Other Observable Inputs (Level 2) [Member]
Mar. 31, 2012
Corporate Debt Securities [Member]
Significant Unobservable Inputs (Level 3) [Member]
Dec. 31, 2011
Corporate Debt Securities [Member]
Significant Unobservable Inputs (Level 3) [Member]
Mar. 31, 2012
Commercial Mortgage-Backed Securities [Member]
Dec. 31, 2011
Commercial Mortgage-Backed Securities [Member]
Mar. 31, 2012
Commercial Mortgage-Backed Securities [Member]
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Dec. 31, 2011
Commercial Mortgage-Backed Securities [Member]
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Mar. 31, 2012
Commercial Mortgage-Backed Securities [Member]
Significant Other Observable Inputs (Level 2) [Member]
Dec. 31, 2011
Commercial Mortgage-Backed Securities [Member]
Significant Other Observable Inputs (Level 2) [Member]
Mar. 31, 2012
Commercial Mortgage-Backed Securities [Member]
Significant Unobservable Inputs (Level 3) [Member]
Dec. 31, 2011
Commercial Mortgage-Backed Securities [Member]
Significant Unobservable Inputs (Level 3) [Member]
Mar. 31, 2012
Residential Mortgage-Backed Securities [Member]
Dec. 31, 2011
Residential Mortgage-Backed Securities [Member]
Mar. 31, 2012
Residential Mortgage-Backed Securities [Member]
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Dec. 31, 2011
Residential Mortgage-Backed Securities [Member]
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Mar. 31, 2012
Residential Mortgage-Backed Securities [Member]
Significant Other Observable Inputs (Level 2) [Member]
Dec. 31, 2011
Residential Mortgage-Backed Securities [Member]
Significant Other Observable Inputs (Level 2) [Member]
Mar. 31, 2012
Residential Mortgage-Backed Securities [Member]
Significant Unobservable Inputs (Level 3) [Member]
Dec. 31, 2011
Residential Mortgage-Backed Securities [Member]
Significant Unobservable Inputs (Level 3) [Member]
Mar. 31, 2012
Debt Securities Issued by Foreign Sovereign Governments [Member]
Dec. 31, 2011
Debt Securities Issued by Foreign Sovereign Governments [Member]
Mar. 31, 2012
Debt Securities Issued by Foreign Sovereign Governments [Member]
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Dec. 31, 2011
Debt Securities Issued by Foreign Sovereign Governments [Member]
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Mar. 31, 2012
Debt Securities Issued by Foreign Sovereign Governments [Member]
Significant Other Observable Inputs (Level 2) [Member]
Dec. 31, 2011
Debt Securities Issued by Foreign Sovereign Governments [Member]
Significant Other Observable Inputs (Level 2) [Member]
Mar. 31, 2012
Debt Securities Issued by Foreign Sovereign Governments [Member]
Significant Unobservable Inputs (Level 3) [Member]
Dec. 31, 2011
Debt Securities Issued by Foreign Sovereign Governments [Member]
Significant Unobservable Inputs (Level 3) [Member]
Mar. 31, 2012
Total Debt Securities [Member]
Dec. 31, 2011
Total Debt Securities [Member]
Mar. 31, 2012
Total Debt Securities [Member]
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Dec. 31, 2011
Total Debt Securities [Member]
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Mar. 31, 2012
Total Debt Securities [Member]
Significant Other Observable Inputs (Level 2) [Member]
Dec. 31, 2011
Total Debt Securities [Member]
Significant Other Observable Inputs (Level 2) [Member]
Mar. 31, 2012
Total Debt Securities [Member]
Significant Unobservable Inputs (Level 3) [Member]
Dec. 31, 2011
Total Debt Securities [Member]
Significant Unobservable Inputs (Level 3) [Member]
Mar. 31, 2012
Equity Securities [Member]
Dec. 31, 2011
Equity Securities [Member]
Mar. 31, 2012
Equity Securities [Member]
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Dec. 31, 2011
Equity Securities [Member]
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Mar. 31, 2012
Equity Securities [Member]
Significant Other Observable Inputs (Level 2) [Member]
Dec. 31, 2011
Equity Securities [Member]
Significant Other Observable Inputs (Level 2) [Member]
Mar. 31, 2012
Equity Securities [Member]
Significant Unobservable Inputs (Level 3) [Member]
Dec. 31, 2011
Equity Securities [Member]
Significant Unobservable Inputs (Level 3) [Member]
Fair value measurements [Abstract]                                                                                                                                                                    
Discount rates used for the DCF model, minimum (in hundredths) 2.24%                                                                                                                                                                  
Discount rate used for the DCF model, maximum (in hundredths) 4.24%                                                                                                                                                                  
Percentage of change in discount rate (in hundredths) 1.00%                                                                                                                                                                  
Change in value of ARS with change in discount rate assumption $ 3,000,000                                                                                                                                                                  
Change in years to liquidity assumption 2Y                                                                                                                                                                  
Change in value of ARS with change in expected term 4,500,000                                                                                                                                                                  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                                                                                                                                                                    
Total investments 5,536,249,000 5,823,647,000                     385,925,000 748,894,000 5,003,369,000 4,899,978,000 146,955,000 174,775,000 237,049,000 597,037,000 237,049,000 597,037,000 0 0 0 0 1,658,469,000 2,323,471,000 0 0 1,562,953,000 2,209,245,000 95,516,000 114,226,000 2,772,447,000 2,032,851,000 0 1,455,000 2,721,329,000 1,971,168,000 51,118,000 60,228,000 261,936,000 264,934,000 0 0 261,936,000 264,934,000 0 0 457,151,000 445,417,000 0 0 457,151,000 445,417,000 0 0 146,414,000 157,190,000 146,414,000 147,976,000 0 9,214,000 0 0 5,533,466,000 5,820,900,000 383,463,000 746,468,000 5,003,369,000 4,899,978,000 146,634,000 174,454,000 2,783,000 2,747,000 2,462,000 2,426,000 0 0 321,000 321,000
Real estate acquired 2,340,000 [1] 1,621,000 [1]                     0 [1] 0 [1] 0 [1] 0 [1] 2,340,000 [1] 1,621,000 [1]                                                                                                                                
Total realized/unrealized gains (losses) [Abstract]                                                                                                                                                                    
Balance at beginning of period     114,226,000 295,690,000 60,228,000 70,053,000 321,000 321,000 174,775,000 366,064,000 1,621,000 6,220,000                                                                                                                                            
Included in earnings and reported as realized investment losses, net     (1,950,000)   (381,000)   0   (2,331,000)   0                                                                                                                                              
Included in earnings and reported as losses incurred, net     0 0 0 0 0 0 0 0 (316,000) 7,000                                                                                                                                            
Included in other comprehensive income     1,869,000 533,000 277,000 220,000 0 0 2,146,000 753,000 0 0                                                                                                                                            
Purchases     27,000 0 0 0 0 0 27,000 0 2,082,000 1,369,000                                                                                                                                            
Sales     (18,656,000) (25,492,000) (9,006,000) 0 0 0 (27,662,000) (25,492,000) (1,047,000) (2,720,000)                                                                                                                                            
Transfers into Level 3     0 0 0 0 0 0 0 0 0 0                                                                                                                                            
Transfers out of Level 3     0 0 0 0 0 0 0 0 0 0                                                                                                                                            
Balance at end of period     95,516,000 270,731,000 51,118,000 70,273,000 321,000 321,000 146,955,000 341,325,000 2,340,000 4,876,000                                                                                                                                            
Amount of total losses included in earnings for the period attributable to the change in unrealized losses on assets still held at period end     $ 0 $ 0 $ 0 $ 0 $ 0 $ 0 $ 0 $ 0 $ 0 $ 0                                                                                                                                            
[1] Real estate acquired through claim settlement, which is held for sale, is reported in Other Assets on the consolidated balance sheet.