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Segment Information (Income From Continuing Operations, Before and After Federal Income Tax) (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2020
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Net investment income $ 68,511 $ 68,185 $ 34,444 $ 55,967 $ 57,594 $ 55,826 $ 58,505 $ 50,618 $ 227,107 $ 222,543 $ 195,336
Net realized and unrealized investment (losses) gain $ 20,079 $ 7,721 $ 12,649 $ (44,666) $ (873) $ (2,183) $ 4,027 $ 13,451 (4,217) 14,422 (54,923)
Tax on investment income                 41,609 45,301 19,560
Total investment income, after federal income tax                 181,281 191,664 120,853
Total investment revenues                 222,890 236,965 140,413
Standard Commercial Lines [Member]                      
Underwriting gain (loss), before federal income tax                 151,731 145,990 109,104
Underwriting gain (loss), after federal income tax                 $ 119,867 $ 115,332 $ 86,192
Combined Ratio                 92.90% 92.90% 94.30%
Return on Equity                 5.10% 5.80% 4.90%
Standard Personal Lines                      
Underwriting gain (loss), before federal income tax                 $ (15,508) $ 8,260 $ 12,764
Underwriting gain (loss), after federal income tax                 $ (12,251) $ 6,525 $ 10,084
Combined Ratio                 105.20% 97.30% 95.80%
Return on Equity                 (0.50%) 0.30% 0.60%
E&S Lines [Member]                      
Underwriting gain (loss), before federal income tax                 $ 126 $ 9,743 $ (695)
Underwriting gain (loss), after federal income tax                 $ 100 $ 7,697 $ (549)
Combined Ratio                 99.90% 95.90% 100.30%
Return on Equity                 0.00% 0.40% 0.00%
Investments SIGI                      
Return on Equity                 7.80% 9.60% 6.90%