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Fair Value Measurements (Quantitative Disclosures of Fair Value Assets) (Details) (USD $) In Thousands, unless otherwise specified | Sep. 30, 2012 | Dec. 31, 2011
--- | --- | ---
AFS fixed maturity securities | $ 3,269,960 |  | $ 2,897,373 |
Equity securities, AFS | 155,577 |  | 157,355 |
Short-term investments | 184,878 |  | 217,044 |
Fair Value [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
AFS fixed maturity securities | 3,269,960 |  | 2,897,373 |
Equity securities, AFS | 155,577 |  | 157,355 |
Short-term investments | 184,878 |  | 217,044 |
Receivable for proceeds related to sale of Selective HR | 2,799 |  | 3,212 |
Total assets | 3,613,214 |  | 3,274,984 |
Fair Value [Member] | US Government and Government Agencies and Authorities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
AFS fixed maturity securities | 294,214 | [1] | 353,796 | [1]
Fair Value [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
AFS fixed maturity securities | 30,292 |  | 34,173 |
Fair Value [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
AFS fixed maturity securities | 784,501 |  | 622,659 |
Fair Value [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
AFS fixed maturity securities | 1,425,888 |  | 1,213,310 |
Fair Value [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
AFS fixed maturity securities | 121,154 |  | 78,949 |
Fair Value [Member] | Commercial Mortgage-backed Securities ("CMBS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
AFS fixed maturity securities | 129,615 |  | 112,598 |
Fair Value [Member] | Residential Mortgage Backed Securities ("RMBS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
AFS fixed maturity securities | 484,296 |  | 481,888 |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
AFS fixed maturity securities | 130,435 | [2] | 126,475 | [2]
Equity securities, AFS | 151,970 | [2] | 157,355 | [2]
Short-term investments | 184,878 | [2] | 217,044 | [2]
Receivable for proceeds related to sale of Selective HR | 0 | [2] | 0 | [2]
Total assets | 467,283 | [2] | 500,874 | [2]
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | US Government and Government Agencies and Authorities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
AFS fixed maturity securities | 130,435 | [1],[2] | 126,475 | [1],[2]
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
AFS fixed maturity securities | 0 | [2] | 0 | [2]
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
AFS fixed maturity securities | 0 | [2] | 0 | [2]
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
AFS fixed maturity securities | 0 | [2] | 0 | [2]
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
AFS fixed maturity securities | 0 | [2] | 0 | [2]
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Commercial Mortgage-backed Securities ("CMBS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
AFS fixed maturity securities | 0 | [2] | 0 | [2]
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Residential Mortgage Backed Securities ("RMBS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
AFS fixed maturity securities | 0 | [2] | 0 | [2]
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
AFS fixed maturity securities | 3,094,637 | [2] | 2,746,200 | [2]
Equity securities, AFS | 0 | [2] | 0 | [2]
Short-term investments | 0 | [2] | 0 | [2]
Receivable for proceeds related to sale of Selective HR | 0 | [2] | 0 | [2]
Total assets | 3,094,637 | [2] | 2,746,200 | [2]
Significant Other Observable Inputs (Level 2) [Member] | US Government and Government Agencies and Authorities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
AFS fixed maturity securities | 143,804 | [1],[2] | 205,580 | [1],[2]
Significant Other Observable Inputs (Level 2) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
AFS fixed maturity securities | 30,292 | [2] | 34,173 | [2]
Significant Other Observable Inputs (Level 2) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
AFS fixed maturity securities | 784,501 | [2] | 622,659 | [2]
Significant Other Observable Inputs (Level 2) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
AFS fixed maturity securities | 1,423,005 | [2] | 1,210,707 | [2]
Significant Other Observable Inputs (Level 2) [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
AFS fixed maturity securities | 113,835 | [2] | 78,949 | [2]
Significant Other Observable Inputs (Level 2) [Member] | Commercial Mortgage-backed Securities ("CMBS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
AFS fixed maturity securities | 114,904 | [2] | 112,244 | [2]
Significant Other Observable Inputs (Level 2) [Member] | Residential Mortgage Backed Securities ("RMBS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
AFS fixed maturity securities | 484,296 | [2] | 481,888 | [2]
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
AFS fixed maturity securities | 44,888 |  | 24,698 |
Equity securities, AFS | 3,607 |  | 0 |
Short-term investments | 0 |  | 0 |
Receivable for proceeds related to sale of Selective HR | 2,799 |  | 3,212 |
Total assets | 51,294 |  | 27,910 |
Significant Unobservable Inputs (Level 3) [Member] | US Government and Government Agencies and Authorities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
AFS fixed maturity securities | 19,975 | [1] | 21,741 | [1]
Significant Unobservable Inputs (Level 3) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
AFS fixed maturity securities | 0 |  | 0 |
Significant Unobservable Inputs (Level 3) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
AFS fixed maturity securities | 0 |  | 0 |
Significant Unobservable Inputs (Level 3) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
AFS fixed maturity securities | 2,883 |  | 2,603 |
Significant Unobservable Inputs (Level 3) [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
AFS fixed maturity securities | 7,319 |  | 0 |
Significant Unobservable Inputs (Level 3) [Member] | Commercial Mortgage-backed Securities ("CMBS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
AFS fixed maturity securities | 14,711 |  | 354 |
Significant Unobservable Inputs (Level 3) [Member] | Residential Mortgage Backed Securities ("RMBS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
AFS fixed maturity securities | $ 0 |  | $ 0 |
[1] | 2 U.S. government includes corporate securities fully guaranteed by the FDIC.
[2] | 1 There were no transfers of securities between Level 1 and Level 2.

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