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Investments (Credit Concentration Risk) (Details) | Sep. 30, 2018 | Dec. 31, 2017
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Investments [Abstract] |  |
Maximum exposure to credit concentration risk of the Company's stockholder's equity other than certain U.S. government-backed investments | 10.00% | 10.00%

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- Definition Maximum exposure to credit concentration risk of the Company's stockholder's equity other than certain U.S. government agencies + References No definition available. + Details
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- References No definition available. + Details
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