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Investments (Fair Value Below Amortized Cost of Gross Unrealized Losses for Fixed Maturity and Equity Securities) (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 | Dec. 31, 2017
Gross unrealized losses with number of securities [Line Items] |  |
Gross Unrealized Losses | $ 381,465 | $ 116,993
% of Total | 100.00% | 100.00%
Less than 20% |  |
Gross unrealized losses with number of securities [Line Items] |  |
Gross Unrealized Losses | $ 358,873 | $ 113,466
% of Total | 94.10% | 97.00%
20% or more for less than six months |  |
Gross unrealized losses with number of securities [Line Items] |  |
Gross Unrealized Losses | $ 20,232 | $ 689
% of Total | 5.30% | 0.60%
20% or more for six months or greater |  |
Gross unrealized losses with number of securities [Line Items] |  |
Gross Unrealized Losses | $ 2,360 | $ 2,838
% of Total | 0.60% | 2.40%

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- Definition Amount of unrealized loss before deducting unrealized gain on investments in available-for-sale securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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