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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%

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