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Segment Information (Income From Continuing Operations, Before and After Federal Income Tax) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Net investment income $ 326,589 $ 227,107 $ 222,543
Net realized and unrealized investment gains (losses) 17,599 (4,217) 14,422
Tax on investment income 67,284 41,609 45,301
Total investment income, after federal income tax 276,904 181,281 191,664
Total investment revenues 344,188 222,890 236,965
Standard Commercial Lines [Member]      
Underwriting gain (loss), before federal income tax 198,596 151,731 145,990
Underwriting gain (loss), after federal income tax $ 156,891 $ 119,867 $ 115,332
Combined Ratio 91.90% 92.90% 92.90%
Return on Equity 5.90% 5.10% 5.80%
Standard Personal Lines      
Underwriting gain (loss), before federal income tax $ 3,966 $ (15,508) $ 8,260
Underwriting gain (loss), after federal income tax $ 3,133 $ (12,251) $ 6,525
Combined Ratio 98.60% 105.20% 97.30%
Return on Equity 0.10% (0.50%) 0.30%
E&S Lines [Member]      
Underwriting gain (loss), before federal income tax $ 16,030 $ 126 $ 9,743
Underwriting gain (loss), after federal income tax $ 12,664 $ 100 $ 7,697
Combined Ratio 94.30% 99.90% 95.90%
Return on Equity 0.50% 0.00% 0.40%
Investments SIGI      
Return on Equity 9.90% 7.80% 9.60%