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Fair Value Measurements (Quantitative Disclosures of Fair Value Assets) (Details) (USD $) In Thousands, unless otherwise specified | Mar. 31, 2013 | Dec. 31, 2012
--- | --- | ---
Available-for-sale Secuities | $ 3,465,981 |  | $ 3,296,013 |
Equity securities, AFS | 174,745 |  | 151,382 |
Short-term investments | 163,440 |  | 214,479 |
Fair Value [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 3,465,981 |  | 3,296,013 |
Equity securities, AFS | 174,745 |  | 151,382 |
Short-term investments | 163,440 |  | 214,479 |
Receivable for proceeds related to sale of Selective HR |  |  | 2,705 |
Total assets | 3,804,166 |  | 3,664,579 |
Fair Value [Member] | US Government and Government Agencies and Authorities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 245,664 |  | 259,092 |
Fair Value [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 30,163 |  | 30,229 |
Fair Value [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 849,946 |  | 818,024 |
Fair Value [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 1,532,753 |  | 1,450,247 |
Fair Value [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 171,943 |  | 128,640 |
Fair Value [Member] | Commercial Mortgage-backed Securities ("CMBS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 146,540 |  | 137,119 |
Fair Value [Member] | Residential Mortgage Backed Securities ("RMBS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 488,972 |  | 472,662 |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 104,507 | [1] | 115,861 | [1]
Equity securities, AFS | 167,203 | [1] | 147,775 | [1]
Short-term investments | 163,440 | [1] | 214,479 | [1]
Receivable for proceeds related to sale of Selective HR |  |  | 0 | [1]
Total assets | 435,150 | [1] | 478,115 | [1]
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] |  |  |  |
Available-for-sale Secuities | 104,507 | [1] | 115,861 | [1]
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 0 | [1] | 0 | [1]
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 0 | [1] | 0 | [1]
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 0 | [1] | 0 | [1]
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 0 | [1] | 0 | [1]
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Commercial Mortgage-backed Securities ("CMBS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 0 | [1] | 0 | [1]
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Residential Mortgage Backed Securities ("RMBS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 0 | [1] | 0 | [1]
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 3,327,183 | [1] | 3,144,187 | [1]
Equity securities, AFS | 4,642 | [1] | 0 | [1]
Short-term investments | 0 | [1] | 0 | [1]
Receivable for proceeds related to sale of Selective HR |  |  | 0 | [1]
Total assets | 3,331,825 | [1] | 3,144,187 | [1]
Significant Other Observable Inputs (Level 2) [Member] | US Government and Government Agencies and Authorities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 122,185 | [1] | 123,442 | [1]
Significant Other Observable Inputs (Level 2) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 30,163 | [1] | 30,229 | [1]
Significant Other Observable Inputs (Level 2) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 849,946 | [1] | 818,024 | [1]
Significant Other Observable Inputs (Level 2) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 1,529,851 | [1] | 1,447,301 | [1]
Significant Other Observable Inputs (Level 2) [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 165,887 | [1] | 122,572 | [1]
Significant Other Observable Inputs (Level 2) [Member] | Commercial Mortgage-backed Securities ("CMBS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 140,179 | [1] | 129,957 | [1]
Significant Other Observable Inputs (Level 2) [Member] | Residential Mortgage Backed Securities ("RMBS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 488,972 | [1] | 472,662 | [1]
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 34,291 |  | 35,965 |
Equity securities, AFS | 2,900 |  | 3,607 |
Short-term investments | 0 |  | 0 |
Receivable for proceeds related to sale of Selective HR |  |  | 2,705 |
Total assets | 37,191 |  | 42,277 |
Significant Unobservable Inputs (Level 3) [Member] | US Government and Government Agencies and Authorities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 18,972 |  | 19,789 |
Significant Unobservable Inputs (Level 3) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 0 |  | 0 |
Significant Unobservable Inputs (Level 3) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 0 |  | 0 |
Significant Unobservable Inputs (Level 3) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 2,902 |  | 2,946 |
Significant Unobservable Inputs (Level 3) [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 6,056 |  | 6,068 |
Significant Unobservable Inputs (Level 3) [Member] | Commercial Mortgage-backed Securities ("CMBS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 6,361 |  | 7,162 |
Significant Unobservable Inputs (Level 3) [Member] | Residential Mortgage Backed Securities ("RMBS") [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | $ 0 |  | $ 0 |
[1] | 1 There were no transfers of securities between Level 1 and Level 2.

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