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Summary of Significant Accounting Policies (Narrative) (Detail) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |
Cash on hand or on deposit required | $ 179.6 | $ 176.6
Cash collateral on deposit | 19.6 | 14.7
Foreclosed Assets | $ 2.1 | $ 2.4
Minimum [Member] |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |
Range of change in fair value of derivative hedging instrument to be considered highly effective | 80.00% |
Maximum [Member] |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |
Range of change in fair value of derivative hedging instrument to be considered highly effective | 125.00% |

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