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Investments - Narrative (Details) - USD ($) $ in Thousands | Sep. 30, 2017 | Dec. 31, 2016
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Investment Holdings [Line Items] |  |
Investment Grade Rate | 92.00% |
Fair value of issued securities | $ 7,630,634 | $ 7,456,708
Federal Home Loan Bank Funding Agreements | 575,000 | 575,000
Federal Home Loans Bank Of Chicago |  |
Investment Holdings [Line Items] |  |
Fair value of issued securities | 620,558 | 620,489
Governmental Agencies as Required by Law in Various States |  |
Investment Holdings [Line Items] |  |
Fair value of issued securities | $ 18,133 | $ 18,119

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