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Fair Value Measurements (Quantitative Disclosures of Fair Value Assets) (Details) (USD $) In Thousands, unless otherwise specified | Mar. 31, 2012 | Dec. 31, 2011
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fixed maturity securities, AFS | $ 3,034,511 |  | $ 2,897,373 |
Equity securities, AFS | 152,986 |  | 157,355 |
Short-term investments | 174,472 |  | 217,044 |
Fair Value [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fixed maturity securities, AFS | 3,034,511 |  | 2,897,373 |
Equity securities, AFS | 152,986 |  | 157,355 |
Short-term investments | 174,472 |  | 217,044 |
Receivable for proceeds related to sale of Selective HR | 2,989 |  | 3,212 |
Total financial assets measured on a recurring basis | 3,364,958 |  | 3,274,984 |
Fair Value [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fixed maturity securities, AFS | 335,180 | [1] | 353,796 | [1]
Fair Value [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fixed maturity securities, AFS | 37,817 |  | 34,173 |
Fair Value [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fixed maturity securities, AFS | 669,073 |  | 622,659 |
Fair Value [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fixed maturity securities, AFS | 1,294,945 |  | 1,213,310 |
Fair Value [Member] | Asset-backed Securities ("ABS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fixed maturity securities, AFS | 94,742 |  | 78,949 |
Fair Value [Member] | Commercial Mortgage-backed Securities ("CMBS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fixed maturity securities, AFS | 112,048 |  | 112,598 |
Fair Value [Member] | Residential Mortgage Backed Securities ("RMBS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fixed maturity securities, AFS | 490,706 |  | 481,888 |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fixed maturity securities, AFS | 130,189 | [2] | 126,475 |
Equity securities, AFS | 152,986 | [2] | 157,355 |
Short-term investments | 174,472 | [2] | 217,044 |
Receivable for proceeds related to sale of Selective HR | 0 | [2] | 0 |
Total financial assets measured on a recurring basis | 457,647 | [2] | 500,874 |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fixed maturity securities, AFS | 130,189 | [1],[2] | 126,475 | [1]
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fixed maturity securities, AFS | 0 | [2] | 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] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fixed maturity securities, AFS | 0 | [2] | 0 |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fixed maturity securities, AFS | 0 | [2] | 0 |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Asset-backed Securities ("ABS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fixed maturity securities, AFS | 0 | [2] | 0 |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Commercial Mortgage-backed Securities ("CMBS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fixed maturity securities, AFS | 0 | [2] | 0 |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Residential Mortgage Backed Securities ("RMBS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fixed maturity securities, AFS | 0 | [2] | 0 |
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fixed maturity securities, AFS | 2,880,206 | [2] | 2,746,200 |
Equity securities, AFS | 0 | [2] | 0 |
Short-term investments | 0 | [2] | 0 |
Receivable for proceeds related to sale of Selective HR | 0 | [2] | 0 |
Total financial assets measured on a recurring basis | 2,880,206 | [2] | 2,746,200 |
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fixed maturity securities, AFS | 183,768 | [1],[2] | 205,580 | [1]
Significant Other Observable Inputs (Level 2) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fixed maturity securities, AFS | 37,817 | [2] | 34,173 |
Significant Other Observable Inputs (Level 2) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fixed maturity securities, AFS | 669,073 | [2] | 622,659 |
Significant Other Observable Inputs (Level 2) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fixed maturity securities, AFS | 1,292,433 | [2] | 1,210,707 |
Significant Other Observable Inputs (Level 2) [Member] | Asset-backed Securities ("ABS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fixed maturity securities, AFS | 94,742 | [2] | 78,949 |
Significant Other Observable Inputs (Level 2) [Member] | Commercial Mortgage-backed Securities ("CMBS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fixed maturity securities, AFS | 111,667 | [2] | 112,244 |
Significant Other Observable Inputs (Level 2) [Member] | Residential Mortgage Backed Securities ("RMBS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fixed maturity securities, AFS | 490,706 | [2] | 481,888 |
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fixed maturity securities, AFS | 24,116 |  | 24,698 |
Equity securities, AFS | 0 |  | 0 |
Short-term investments | 0 |  | 0 |
Receivable for proceeds related to sale of Selective HR | 2,989 |  | 3,212 |
Total financial assets measured on a recurring basis | 27,105 |  | 27,910 |
Significant Unobservable Inputs (Level 3) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fixed maturity securities, AFS | 21,223 | [1] | 21,741 | [1]
Significant Unobservable Inputs (Level 3) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fixed maturity securities, AFS | 0 |  | 0 |
Significant Unobservable Inputs (Level 3) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fixed maturity securities, AFS | 0 |  | 0 |
Significant Unobservable Inputs (Level 3) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fixed maturity securities, AFS | 2,512 |  | 2,603 |
Significant Unobservable Inputs (Level 3) [Member] | Asset-backed Securities ("ABS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fixed maturity securities, AFS | 0 |  | 0 |
Significant Unobservable Inputs (Level 3) [Member] | Commercial Mortgage-backed Securities ("CMBS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fixed maturity securities, AFS | 381 |  | 354 |
Significant Unobservable Inputs (Level 3) [Member] | Residential Mortgage Backed Securities ("RMBS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fixed maturity securities, AFS | $ 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 in First Quarter 2012.

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