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Segment Information (Income Before and After Federal Income Tax) (Details) - USD ($) $ in Thousands | 3 Months Ended
--- | ---
Mar. 31, 2021 | Mar. 31, 2020
Segment Reporting Information [Line Items] |  |
Net investment income | $ 69,716 | $ 55,967
Net realized and unrealized investment gains (losses) | 5,119 | (44,666)
Total investment segment income, before federal income tax | 74,835 | 11,301
Tax on investment income | 14,448 | 1,104
Total investment segment income, after federal income tax | 60,387 | 10,197
Standard Commercial Lines |  |
Segment Reporting Information [Line Items] |  |
Underwriting gain, before federal income tax | 69,499 | 17,126
Underwriting gain, after federal income tax | $ 54,904 | $ 13,529
Combined ratio | 88.20% | 96.70%
ROE Contribution | 8.60% | 2.40%
Standard Personal Lines |  |
Segment Reporting Information [Line Items] |  |
Underwriting gain, before federal income tax | $ 7,695 | $ 387
Underwriting gain, after federal income tax | $ 6,079 | $ 306
Combined ratio | 89.60% | 99.50%
ROE Contribution | 1.00% | 0.10%
E&S Lines |  |
Segment Reporting Information [Line Items] |  |
Underwriting gain, before federal income tax | $ 516 | $ 3,844
Underwriting gain, after federal income tax | $ 408 | $ 3,037
Combined ratio | 99.20% | 93.50%
ROE Contribution | 0.10% | 0.60%
Investments |  |
Segment Reporting Information [Line Items] |  |
ROE Contribution | 8.90% | 8.50%

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