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Fair Value of Assets and Liabilities - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Dec. 31, 2017
Sep. 30, 2017
Mar. 31, 2016
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Segment Reporting Information [Line Items]                  
Gain/(loss) on instrument specific credit risk             $ 0.2 $ 0.5 $ 1.5
Corporate                  
Segment Reporting Information [Line Items]                  
Long-lived asset impairment $ 1.9 $ 0.7 $ 1.3 $ 3.0 $ 2.0        
Reversed asset impairment charges $ 0.5 $ 1.0              
Regional Banking                  
Segment Reporting Information [Line Items]                  
Long-lived asset impairment       $ 0.8   $ 3.7