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Segment Information (Income Before and After Federal Income Tax) (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Segment Reporting Information [Line Items] |  |  |  |
Net investment income | $ 52,443 | $ 40,446 | $ 141,227 | $ 119,295
Net realized and unrealized investment (losses) gains | (4,787) | 6,798 | (16,988) | 7,487
Total investment income, before federal income tax | 47,656 | 47,244 | 124,239 | 126,782
Tax on investment income | 8,562 | 13,236 | 21,405 | 34,572
Total investment income, after federal income tax | 39,094 | 34,008 | 102,834 | 92,210
Standard Commercial Lines [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Underwriting gain (loss), before federal income tax | 26,333 | 35,329 | 74,153 | 112,634
Underwriting gain (loss), after federal income tax | $ 20,803 | $ 22,964 | $ 58,581 | $ 73,212
Combined ratio | 94.50% | 92.10% | 94.80% | 91.50%
Standard Personal Lines [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Underwriting gain (loss), before federal income tax | $ 3,158 | $ 8,179 | $ 6,457 | $ 7,517
Underwriting gain (loss), after federal income tax | $ 2,495 | $ 5,316 | $ 5,101 | $ 4,886
Combined ratio | 95.90% | 88.70% | 97.20% | 96.50%
Excess And Surplus Operations [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Underwriting gain (loss), before federal income tax | $ 3,506 | $ (11,063) | $ (4,848) | $ (8,174)
Underwriting gain (loss), after federal income tax | $ 2,770 | $ (7,191) | $ (3,830) | $ (5,313)
Combined ratio | 93.70% | 120.40% | 103.00% | 105.20%

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