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Fair Value Measurements (Quantitative Disclosures of Fair Value Assets) (Details) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2011 | Dec. 31, 2010 | Jun. 30, 2009
--- | --- | --- | ---
Fixed maturity securities, AFS | $ 2,897,373 |  | $ 2,342,742 |  |
Equity securities, AFS | 157,355 |  | 69,636 |  | 31,100
Short-term investments | 217,044 |  | 161,155 |  |
Fair Value [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |  |
Fixed maturity securities, AFS | 2,897,373 |  | 2,342,742 |  |
Equity securities, AFS | 157,355 |  | 69,636 |  |
Short-term investments | 217,044 |  | 161,155 |  |
Receivable for proceeds related to sale of Selective HR | 3,212 |  | 5,002 |  |
Total financial assets measured on a recurring basis | 3,274,984 |  | 2,578,535 |  |
Fair Value [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |  |
Fixed maturity securities, AFS | 353,796 | [1] | 320,529 | [1] |
Fair Value [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |  |
Fixed maturity securities, AFS | 34,173 |  | 18,966 |  |
Fair Value [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |  |
Fixed maturity securities, AFS | 622,659 |  | 533,897 |  |
Fair Value [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |  |
Fixed maturity securities, AFS | 1,213,310 |  | 993,725 |  |
Fair Value [Member] | Asset-backed Securities ("ABS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |  |
Fixed maturity securities, AFS | 78,949 |  | 48,733 |  |
Fair Value [Member] | Commercial Mortgage-backed Securities ("CMBS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |  |
Fixed maturity securities, AFS | 112,598 |  | 104,475 |  |
Fair Value [Member] | Residential Mortgage Backed Securities ("RMBS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |  |
Fixed maturity securities, AFS | 481,888 |  | 322,417 |  |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |  |
Fixed maturity securities, AFS | 126,475 |  | 105,317 |  |
Equity securities, AFS | 157,355 |  | 69,636 |  |
Short-term investments | 217,044 |  | 161,155 |  |
Receivable for proceeds related to sale of Selective HR | 0 |  | 0 |  |
Total financial assets measured on a recurring basis | 500,874 |  | 336,108 |  |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |  |
Fixed maturity securities, AFS | 126,475 | [1] | 105,317 | [1] |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |  |
Fixed maturity securities, AFS | 0 |  | 0 |  |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |  |
Fixed maturity securities, AFS | 0 |  | 0 |  |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |  |
Fixed maturity securities, AFS | 0 |  | 0 |  |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Asset-backed Securities ("ABS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |  |
Fixed maturity securities, AFS | 0 |  | 0 |  |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Commercial Mortgage-backed Securities ("CMBS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |  |
Fixed maturity securities, AFS | 0 |  | 0 |  |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Residential Mortgage Backed Securities ("RMBS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |  |
Fixed maturity securities, AFS | 0 |  | 0 |  |
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |  |
Fixed maturity securities, AFS | 2,746,200 |  | 2,237,240 |  |
Equity securities, AFS | 0 |  | 0 |  |
Short-term investments | 0 |  | 0 |  |
Receivable for proceeds related to sale of Selective HR | 0 |  | 0 |  |
Total financial assets measured on a recurring basis | 2,746,200 |  | 2,237,240 |  |
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |  |
Fixed maturity securities, AFS | 205,580 | [1] | 215,212 | [1] |
Significant Other Observable Inputs (Level 2) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |  |
Fixed maturity securities, AFS | 34,173 |  | 18,966 |  |
Significant Other Observable Inputs (Level 2) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |  |
Fixed maturity securities, AFS | 622,659 |  | 533,897 |  |
Significant Other Observable Inputs (Level 2) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |  |
Fixed maturity securities, AFS | 1,210,707 |  | 993,725 |  |
Significant Other Observable Inputs (Level 2) [Member] | Asset-backed Securities ("ABS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |  |
Fixed maturity securities, AFS | 78,949 |  | 48,733 |  |
Significant Other Observable Inputs (Level 2) [Member] | Commercial Mortgage-backed Securities ("CMBS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |  |
Fixed maturity securities, AFS | 112,244 |  | 104,290 |  |
Significant Other Observable Inputs (Level 2) [Member] | Residential Mortgage Backed Securities ("RMBS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |  |
Fixed maturity securities, AFS | 481,888 |  | 322,417 |  |
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |  |
Fixed maturity securities, AFS | 24,698 |  | 185 |  |
Equity securities, AFS | 0 |  | 0 |  |
Short-term investments | 0 |  | 0 |  |
Receivable for proceeds related to sale of Selective HR | 3,212 |  | 5,002 |  |
Total financial assets measured on a recurring basis | 27,910 |  | 5,187 |  |
Significant Unobservable Inputs (Level 3) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |  |
Fixed maturity securities, AFS | 21,741 | [1] | 0 | [1] |
Significant Unobservable Inputs (Level 3) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |  |
Fixed maturity securities, AFS | 0 |  | 0 |  |
Significant Unobservable Inputs (Level 3) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |  |
Fixed maturity securities, AFS | 0 |  | 0 |  |
Significant Unobservable Inputs (Level 3) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |  |
Fixed maturity securities, AFS | 2,603 |  | 0 |  |
Significant Unobservable Inputs (Level 3) [Member] | Asset-backed Securities ("ABS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |  |
Fixed maturity securities, AFS | 0 |  | 0 |  |
Significant Unobservable Inputs (Level 3) [Member] | Commercial Mortgage-backed Securities ("CMBS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |  |
Fixed maturity securities, AFS | 354 |  | 185 |  |
Significant Unobservable Inputs (Level 3) [Member] | Residential Mortgage Backed Securities ("RMBS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |  |
Fixed maturity securities, AFS | $ 0 |  | $ 0 |  |
[1] | 1 U.S. government includes corporate securities fully guaranteed by the FDIC.

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