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Fair Value Measurements (Quantitative Disclosures at Fair Value Assets) (Details) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2012 | Dec. 31, 2011
--- | --- | ---
Available-for-sale Securities, Debt Securities | $ 3,296,013 |  | $ 2,897,373 |
Equity securities, AFS | 151,382 |  | 157,355 |
Short-term investments | 214,479 |  | 217,044 |
Fair Value [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Securities, Debt Securities | 3,296,013 |  | 2,897,373 |
Equity securities, AFS | 151,382 |  | 157,355 |
Short-term investments | 214,479 |  | 217,044 |
Receivable for proceeds related to sale of Selective HR | 2,705 |  | 3,212 |
Total Assets | 3,664,579 |  | 3,274,984 |
Fair Value [Member] | U.S. Government and Government Agencies and Authorities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Securities, Debt Securities | 259,092 |  | 353,796 | [1]
Fair Value [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Securities, Debt Securities | 30,229 |  | 34,173 |
Fair Value [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Securities, Debt Securities | 818,024 |  | 622,659 |
Fair Value [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Securities, Debt Securities | 1,450,247 |  | 1,213,310 |
Fair Value [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Securities, Debt Securities | 128,640 |  | 78,949 |
Fair Value [Member] | Commercial Mortgage-backed Securities (CMBS) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Securities, Debt Securities | 137,119 |  | 112,598 |
Fair Value [Member] | Residential Mortgage Backed Securities (RMBS) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Securities, Debt Securities | 472,662 |  | 481,888 |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Securities, Debt Securities | 115,861 | [2] | 126,475 | [2]
Equity securities, AFS | 147,775 | [2] | 157,355 | [2]
Short-term investments | 214,479 | [2] | 217,044 | [2]
Receivable for proceeds related to sale of Selective HR | 0 | [2] | 0 | [2]
Total Assets | 478,115 | [2] | 500,874 | [2]
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | U.S. Government and Government Agencies and Authorities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Securities, Debt Securities | 115,861 | [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] |  |  |  |
Available-for-sale Securities, Debt 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] |  |  |  |
Available-for-sale Securities, Debt 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] |  |  |  |
Available-for-sale Securities, Debt 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] |  |  |  |
Available-for-sale Securities, Debt 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] |  |  |  |
Available-for-sale Securities, Debt 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] |  |  |  |
Available-for-sale Securities, Debt Securities | 0 | [2] | 0 | [2]
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Securities, Debt Securities | 3,144,187 | [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 | 3,144,187 | [2] | 2,746,200 | [2]
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government and Government Agencies and Authorities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Securities, Debt Securities | 123,442 | [2] | 205,580 | [1],[2]
Significant Other Observable Inputs (Level 2) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Securities, Debt Securities | 30,229 | [2] | 34,173 | [2]
Significant Other Observable Inputs (Level 2) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Securities, Debt Securities | 818,024 | [2] | 622,659 | [2]
Significant Other Observable Inputs (Level 2) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Securities, Debt Securities | 1,447,301 | [2] | 1,210,707 | [2]
Significant Other Observable Inputs (Level 2) [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Securities, Debt Securities | 122,572 | [2] | 78,949 | [2]
Significant Other Observable Inputs (Level 2) [Member] | Commercial Mortgage-backed Securities (CMBS) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Securities, Debt Securities | 129,957 | [2] | 112,244 | [2]
Significant Other Observable Inputs (Level 2) [Member] | Residential Mortgage Backed Securities (RMBS) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Securities, Debt Securities | 472,662 | [2] | 481,888 | [2]
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Securities, Debt Securities | 35,965 |  | 24,698 |
Equity securities, AFS | 3,607 |  | 0 |
Short-term investments | 0 |  | 0 |
Receivable for proceeds related to sale of Selective HR | 2,705 |  | 3,212 |
Total Assets | 42,277 |  | 27,910 |
Significant Unobservable Inputs (Level 3) [Member] | U.S. Government and Government Agencies and Authorities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Securities, Debt Securities | 19,789 |  | 21,741 | [1]
Significant Unobservable Inputs (Level 3) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Securities, Debt Securities | 0 |  | 0 |
Significant Unobservable Inputs (Level 3) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Securities, Debt Securities | 0 |  | 0 |
Significant Unobservable Inputs (Level 3) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Securities, Debt Securities | 2,946 |  | 2,603 |
Significant Unobservable Inputs (Level 3) [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Securities, Debt Securities | 6,068 |  | 0 |
Significant Unobservable Inputs (Level 3) [Member] | Commercial Mortgage-backed Securities (CMBS) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Securities, Debt Securities | 7,162 |  | 354 |
Significant Unobservable Inputs (Level 3) [Member] | Residential Mortgage Backed Securities (RMBS) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |  |
Available-for-sale Securities, Debt Securities | $ 0 |  | $ 0 |
[1] | 2 U.S. government includes corporate securities fully guaranteed by the FDIC.
[2] | There were no transfers of securities between Level 1 and Level 2.

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