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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.