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Investments - Narrative (Details) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Investment Holdings [Line Items] |  |
Investment Grade Rate | 95.30% |
Fair value of issued securities | $ 7,634,682 | $ 7,724,075
Federal Home Loan Bank Funding Agreements | 625,000 | 625,000
Federal Home Loans Bank Of Chicago |  |
Investment Holdings [Line Items] |  |
Fair value of issued securities | 686,387 | 686,790
Governmental Agencies as Required by Law in Various States |  |
Investment Holdings [Line Items] |  |
Fair value of issued securities | $ 17,848 | $ 17,985

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