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Basis of Presentation and Significant Accounting Policies FX gain (loss) (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
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Jun. 30, 2019 | Jun. 30, 2018 | Jun. 30, 2019 | Jun. 30, 2018
Foreign Currency Transaction Gain (Loss), before Tax |  |  | $ 13,108 | $ (3,591)
Investments [Member] |  |  |  |
Foreign Currency Transaction Gain (Loss), before Tax |  |  | 23,155 | 2,011
Premiums Receivable [Member] |  |  |  |
Foreign Currency Transaction Gain (Loss), before Tax |  |  | (9,783) | 512
Loss and Loss Reserves [Member] |  |  |  |
Foreign Currency Transaction Gain (Loss), before Tax |  |  | (774) | (6,359)
Gain (Loss) on Investments [Member] |  |  |  |
Foreign Currency Transaction Gain (Loss), before Tax | $ 25,853 | $ 6,815 | $ 23,155 | $ 2,011

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