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Investments (Net Investment Income) (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
Statement [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net investment income earned | $ 42,587 | $ 40,446 | $ 41,430 | $ 37,419 | $ 35,428 | $ 33,375 | $ 31,182 | $ 30,769 | $ 161,882 | $ 130,754 | $ 121,316
Fixed Income Securities [Member] |  |  |  |  |  |  |  |  |  |  |
Statement [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net investment income earned |  |  |  |  |  |  |  |  | 153,230 | 129,306 | 123,230
Equity Securities [Member] |  |  |  |  |  |  |  |  |  |  |
Statement [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net investment income earned |  |  |  |  |  |  |  |  | 6,442 | 7,368 | 9,161
Short-term Investments [Member] |  |  |  |  |  |  |  |  |  |  |
Statement [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net investment income earned |  |  |  |  |  |  |  |  | 1,526 | 686 | 112
Other Investment Portfolio [Member] |  |  |  |  |  |  |  |  |  |  |
Statement [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net investment income earned |  |  |  |  |  |  |  |  | 12,871 | 2,940 | (1,890)
Investment Expenses [Member] |  |  |  |  |  |  |  |  |  |  |
Statement [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net investment income earned |  |  |  |  |  |  |  |  | $ (12,187) | $ (9,546) | $ (9,297)

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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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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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