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Fair Value Measurements (Quantitative Disclosures of Fair Value Assets) (Details) (USD $) In Thousands, unless otherwise specified | Sep. 30, 2013 | Dec. 31, 2012
--- | --- | ---
Available-for-sale Secuities | $ 3,633,432 |  | $ 3,296,013 |
Equity securities, AFS | 180,506 |  | 151,382 |
Short-term investments | 172,087 |  | 214,479 |
Fair Value [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 3,633,432 |  | 3,296,013 |
Equity securities, AFS | 180,506 |  | 151,382 |
Short-term investments | 172,087 |  | 214,479 |
Receivable for proceeds related to sale of Selective HR |  |  | 2,705 |
Total assets | 3,986,025 |  | 3,664,579 |
Fair Value [Member] | US Government and Government Agencies and Authorities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 180,786 |  | 259,092 |
Fair Value [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 29,709 |  | 30,229 |
Fair Value [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 929,498 |  | 818,024 |
Fair Value [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 1,663,151 |  | 1,450,247 |
Fair Value [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 150,766 |  | 128,640 |
Fair Value [Member] | Commercial Mortgage-backed Securities (CMBS) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 151,948 |  | 137,119 |
Fair Value [Member] | Residential Mortgage Backed Securities (RMBS) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 527,574 |  | 472,662 |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 50,871 | [1] | 115,861 | [1]
Equity securities, AFS | 177,606 | [1] | 147,775 | [1]
Short-term investments | 172,087 | [1] | 214,479 | [1]
Receivable for proceeds related to sale of Selective HR |  |  | 0 | [1]
Total assets | 400,564 | [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 | 50,871 | [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 | 3,574,990 | [1] | 3,144,187 | [1]
Equity securities, AFS | 0 | [1] | 0 | [1]
Short-term investments | 0 | [1] | 0 | [1]
Receivable for proceeds related to sale of Selective HR |  |  | 0 | [1]
Total assets | 3,574,990 | [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 | 129,915 | [1] | 123,442 | [1]
Significant Other Observable Inputs (Level 2) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 29,709 | [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 | 929,498 | [1] | 818,024 | [1]
Significant Other Observable Inputs (Level 2) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 1,663,151 | [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 | 144,804 | [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 | 150,339 | [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 | 527,574 | [1] | 472,662 | [1]
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 7,571 |  | 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 | 10,471 |  | 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 | 0 |  | 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 | 0 |  | 2,946 |
Significant Unobservable Inputs (Level 3) [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 5,962 |  | 6,068 |
Significant Unobservable Inputs (Level 3) [Member] | Commercial Mortgage-backed Securities (CMBS) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Secuities | 1,609 |  | 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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