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Segment Information (Income Before and After Federal Income Tax) (Details) - USD ($) $ in Thousands | 3 Months Ended
--- | ---
Mar. 31, 2018 | Mar. 31, 2017
Segment Reporting Information [Line Items] |  |
Net investment income | $ 43,231 | $ 37,419
Net realized and unrealized investment losses | (10,549) | (1,045)
Total investment income, before federal income tax | 32,682 | 36,374
Tax on investment income | 5,226 | 9,602
Total investment income, after federal income tax | 27,456 | 26,772
Standard Commercial Lines [Member] |  |
Segment Reporting Information [Line Items] |  |
Underwriting gain (loss), before federal income tax | 6,804 | 42,546
Underwriting gain (loss), after federal income tax | $ 5,375 | $ 27,655
Combined ratio | 98.50% | 90.30%
Standard Personal Lines [Member] |  |
Segment Reporting Information [Line Items] |  |
Underwriting gain (loss), before federal income tax | $ (1,506) | $ 5,106
Underwriting gain (loss), after federal income tax | $ (1,190) | $ 3,319
Combined ratio | 102.00% | 92.80%
Excess And Surplus Operations [Member] |  |
Segment Reporting Information [Line Items] |  |
Underwriting gain (loss), before federal income tax | $ (565) | $ 1,570
Underwriting gain (loss), after federal income tax | $ (446) | $ 1,021
Combined ratio | 101.10% | 96.90%

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- Definition This item represents the net total realized and unrealized gain (loss) included in earnings for the period as a result of selling or holding marketable securities categorized as trading, available-for-sale, or held-to-maturity, including the unrealized holding gain (loss) of held-to-maturity securities transferred to the trading security category and the cumulative unrealized gain (loss) which was included in other comprehensive income (a separate component of shareholders' equity) for available-for-sale securities transferred to trading securities during the period. Additionally, this item would include any gains (losses) realized during the period from the sale of investments accounted for under the cost method of accounting and losses recognized for other than temporary impairments (OTTI) of the subject investments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (c)-(e) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.7(c),9(a)) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27405-111563 + Details
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