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Segment Information (Income Before Federal Income Tax) (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016
Segment Reporting Information [Line Items] |  |  |  |
Net investment income | $ 40,446 | $ 33,375 | $ 119,295 | $ 95,326
Net realized investment gains | 6,798 | 3,688 | 7,487 | 2,749
Total investment income, before federal income tax | 47,244 | 37,063 | 126,782 | 98,075
Tax on investment income | 13,236 | 9,752 | 34,572 | 24,290
Total investment income, after federal income tax | 34,008 | 27,311 | 92,210 | 73,785
Standard Commercial Lines [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Underwriting gain (loss), before federal income tax | $ 35,329 | $ 30,124 | $ 112,634 | $ 101,229
GAAP combined ratio | 92.10% | 92.90% | 91.50% | 91.80%
Statutory combined ratio | 91.70% | 92.00% | 90.20% | 90.10%
Standard Personal Lines [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Underwriting gain (loss), before federal income tax | $ 8,179 | $ 4,271 | $ 7,517 | $ 19,001
GAAP combined ratio | 88.70% | 93.80% | 96.50% | 90.90%
Statutory combined ratio | 86.40% | 92.00% | 95.20% | 90.70%
Excess And Surplus Operations [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Underwriting gain (loss), before federal income tax | $ (11,063) | $ (2,362) | $ (8,174) | $ (3,465)
GAAP combined ratio | 120.40% | 104.50% | 105.20% | 102.30%
Statutory combined ratio | 120.10% | 101.40% | 104.60% | 100.90%

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- Definition The statutory combined ratio (expressed as a percentage) is the sum of: (i) the loss and loss expense ratio; (ii) the underwriting ratio; and (iii) the dividend ratio. The loss and loss expense ratio as well as the dividend ratio is calculated by dividing loss and loss expenses, as well as dividends to policyholders, by net premiums earned. The underwriting expense ratio is calculated by dividing underwriting expenses by net premiums written. + References No definition available. + Details
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- Definition Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(2)) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-16(Column G)) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574960-122915 + Details
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