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Segment Information (Income From Continuing Operations, Before Federal Income Tax) (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Net investment income | $ 42,587 | $ 40,446 | $ 41,430 | $ 37,419 | $ 35,428 | $ 33,375 | $ 31,182 | $ 30,769 | $ 161,882 | $ 130,754 | $ 121,316
Realized investment gains (losses) |  |  |  |  |  |  |  |  | 6,359 | (4,937) | 13,171
Total investment revenues |  |  |  |  |  |  |  |  | 168,241 | 125,817 | 134,487
Tax on investment income |  |  |  |  |  |  |  |  | 45,588 | 30,621 | 32,090
Total investment income, after federal income tax |  |  |  |  |  |  |  |  | 122,653 | 95,196 | 102,397
Standard Commercial Lines [Member] |  |  |  |  |  |  |  |  |  |  |
Underwriting gain (loss), before federal income tax |  |  |  |  |  |  |  |  | 149,514 | 146,435 | 164,496
Underwriting gain (loss), after federal income tax |  |  |  |  |  |  |  |  | $ 97,184 | $ 95,183 | $ 106,923
Combined Ratio |  |  |  |  |  |  |  |  | 91.60% | 91.20% | 89.20%
Standard Personal Lines [Member] |  |  |  |  |  |  |  |  |  |  |
Underwriting gain (loss), before federal income tax |  |  |  |  |  |  |  |  | $ 11,104 | $ 12,419 | $ 1,336
Underwriting gain (loss), after federal income tax |  |  |  |  |  |  |  |  | $ 7,217 | $ 8,072 | $ 868
Combined Ratio |  |  |  |  |  |  |  |  | 96.20% | 95.60% | 99.50%
Excess And Surplus Operations [Member] |  |  |  |  |  |  |  |  |  |  |
Underwriting gain (loss), before federal income tax |  |  |  |  |  |  |  |  | $ (6,282) | $ (6,921) | $ (16,803)
Underwriting gain (loss), after federal income tax |  |  |  |  |  |  |  |  | $ (4,083) | $ (4,499) | $ (10,922)
Combined Ratio |  |  |  |  |  |  |  |  | 103.00% | 103.40% | 109.80%

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