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Investments, Gain (Loss) on Investments (Details) - USD ($) | 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
Gain (Loss) on Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Recognized other than temporary impairments |  |  |  |  |  |  |  |  | $ 0 | $ 0 | $ 0
Realized gains (losses) | $ 460,000 | $ (47,000) | $ (42,000) | $ (122,000) | $ (52,000) | $ 5,092,000 | $ 836,000 | $ 3,056,000 | 249,000 | 8,932,000 | 28,361,000
Change in unrealized gains (losses) |  |  |  |  |  |  |  |  | 69,052,000 | (5,425,000) | (33,718,000)
Gross realized gains, gross realized losses and impairment losses [Abstract] |  |  |  |  |  |  |  |  |  |  |
Gross realized gains |  |  |  |  |  |  |  |  | 1,599,000 | 11,909,000 | 30,039,000
Gross realized losses |  |  |  |  |  |  |  |  | (1,350,000) | (2,977,000) | (1,678,000)
Net realized investment gains | $ 460,000 | $ (47,000) | $ (42,000) | $ (122,000) | $ (52,000) | $ 5,092,000 | $ 836,000 | $ 3,056,000 | 249,000 | 8,932,000 | 28,361,000
Available-for-Sale Securities |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Realized gains (losses) |  |  |  |  |  |  |  |  | 249,000 | 8,932,000 | 28,361,000
Gross realized gains, gross realized losses and impairment losses [Abstract] |  |  |  |  |  |  |  |  |  |  |
Net realized investment gains |  |  |  |  |  |  |  |  | 249,000 | 8,932,000 | 28,361,000
Available-for-Sale Securities | Fixed income |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Realized gains (losses) |  |  |  |  |  |  |  |  | 228,000 | 5,310,000 | 28,335,000
Change in unrealized gains (losses) |  |  |  |  |  |  |  |  | 69,026,000 | (5,403,000) | (33,687,000)
Gross realized gains, gross realized losses and impairment losses [Abstract] |  |  |  |  |  |  |  |  |  |  |
Net realized investment gains |  |  |  |  |  |  |  |  | 228,000 | 5,310,000 | 28,335,000
Available-for-Sale Securities | Equity Securities |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Realized gains (losses) |  |  |  |  |  |  |  |  | 21,000 | 3,622,000 | 26,000
Change in unrealized gains (losses) |  |  |  |  |  |  |  |  | 39,000 | (36,000) | (32,000)
Gross realized gains, gross realized losses and impairment losses [Abstract] |  |  |  |  |  |  |  |  |  |  |
Net realized investment gains |  |  |  |  |  |  |  |  | 21,000 | 3,622,000 | 26,000
Available-for-Sale Securities | Other |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Realized gains (losses) |  |  |  |  |  |  |  |  | 0 | 0 | 0
Change in unrealized gains (losses) |  |  |  |  |  |  |  |  | (13,000) | 14,000 | 1,000
Gross realized gains, gross realized losses and impairment losses [Abstract] |  |  |  |  |  |  |  |  |  |  |
Net realized investment gains |  |  |  |  |  |  |  |  | $ 0 | $ 0 | $ 0

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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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