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Derivative Financial Instruments Nature and Amounts of Collateral Received From and Posted To Our Derivative Counterparties (Details) - USD ($) $ in Millions | Mar. 31, 2018 | Dec. 31, 2017
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Derivative |  |
Derivative, Collateral, Obligation to Return Cash | $ 16.1 | $ 15.3
Available-for-sale Securities Pledged as Collateral | 49.6 | 46.4
Total Amount |  |
Derivative |  |
Derivative, Collateral, Obligation to Return Cash | $ 16.2 | $ 15.7

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