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Segment Information (Income Before and After Federal Income Tax) (Details) - USD ($) $ in Thousands | 3 Months Ended
--- | ---
Mar. 31, 2020 | Mar. 31, 2019
Segment Reporting Information [Line Items] |  |
Total investment segment income, after federal income tax | $ 10,197 | $ 51,949
Net investment income | 55,967 | 50,618
Net realized and unrealized investment (losses) gains | (44,666) | 13,451
Total investment segment income, before federal income tax | 11,301 | 64,069
Tax on investment income | 1,104 | 12,120
Standard Commercial Lines [Member] |  |
Segment Reporting Information [Line Items] |  |
Underwriting gain (loss), before federal income tax | 17,126 | 25,815
Underwriting gain (loss), after federal income tax | $ 13,529 | $ 20,394
Combined ratio | 96.70% | 94.80%
ROE Contribution | 2.40% | 4.40%
Standard Personal Lines [Member] |  |
Segment Reporting Information [Line Items] |  |
Underwriting gain (loss), before federal income tax | $ 387 | $ 3,167
Underwriting gain (loss), after federal income tax | $ 306 | $ 2,502
Combined ratio | 99.50% | 95.90%
ROE Contribution | 0.10% | 0.50%
Excess And Surplus Operations [Member] |  |
Segment Reporting Information [Line Items] |  |
Underwriting gain (loss), before federal income tax | $ 3,844 | $ 4,579
Underwriting gain (loss), after federal income tax | $ 3,037 | $ 3,617
Combined ratio | 93.50% | 92.10%
ROE Contribution | 0.60% | 0.80%
Investments [Member] |  |
Segment Reporting Information [Line Items] |  |
ROE Contribution | 8.50% | 8.90%

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