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Investments (Fair Value Below Amortized Cost of Gross Unrealized Losses for Fixed Maturity and Equity Securities) (Details) - USD ($) $ in Thousands | 6 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2018 | Dec. 31, 2017
Gross unrealized losses with number of securities [Line Items] |  |
Gross Unrealized Losses | $ 592,208 | $ 116,993
% of Total | 100.00% | 100.00%
Less than 20% |  |
Gross unrealized losses with number of securities [Line Items] |  |
Gross Unrealized Losses | $ 571,151 | $ 113,466
% of Total | 96.40% | 97.00%
20% or more for less than six months |  |
Gross unrealized losses with number of securities [Line Items] |  |
Gross Unrealized Losses | $ 21,045 | $ 689
% of Total | 3.60% | 0.60%
20% or more for six months or greater |  |
Gross unrealized losses with number of securities [Line Items] |  |
Gross Unrealized Losses | $ 12 | $ 2,838
% of Total | 0.00% | 2.40%

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