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Fair Value of Assets & Liabilities (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Sep. 30, 2017 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Segment Reporting Information [Line Items] |  |  |  |  |  |
Gain/(loss) on instrument specific credit risk |  |  |  | $ 0.5 | $ 1.5 | $ 0.4
Corporate |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |
Long-lived asset impairment | $ 3.0 | $ 2.0 |  |  |  |
Regional Banking |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |
Long-lived asset impairment | $ 0.8 |  | $ 3.7 |  |  |

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