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Investments, Investment Income By Category (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
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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
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |  |  |  |  |  |  |  |
Investment income |  |  |  |  |  |  |  |  | $ 124,378 | $ 113,882 | $ 106,736
Investment expenses |  |  |  |  |  |  |  |  | (3,507) | (3,216) | (2,995)
Net investment income | $ 31,276 | $ 30,402 | $ 29,716 | $ 29,477 | $ 28,094 | $ 27,515 | $ 27,248 | $ 27,809 | 120,871 | 110,666 | 103,741
Fixed income |  |  |  |  |  |  |  |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |  |  |  |  |  |  |  |
Investment income |  |  |  |  |  |  |  |  | 122,105 | 112,513 | 105,882
Equity securities |  |  |  |  |  |  |  |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |  |  |  |  |  |  |  |
Investment income |  |  |  |  |  |  |  |  | 206 | 182 | 208
Cash equivalents |  |  |  |  |  |  |  |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |  |  |  |  |  |  |  |
Investment income |  |  |  |  |  |  |  |  | 1,447 | 754 | 191
Other |  |  |  |  |  |  |  |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |  |  |  |  |  |  |  |
Investment income |  |  |  |  |  |  |  |  | $ 620 | $ 433 | $ 455

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- Definition The income earned from investments in securities and property, equipment and other capital assets. It includes rent from property and equipment, dividends from shares in corporations, and interest from bonds, loans, mortgages, derivatives, commercial paper, bank accounts, certificates of deposits, treasuries, and other financial securities. It does not include realized gains and losses on investments or investment expenses. + References No definition available. + Details
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- Definition Amount of expenses related to the generation of investment income. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.9) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 Reference 2: 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 + 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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- 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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