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Off-Balance Sheet Credit Risk (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]    
General settlement period of securities transactions 2 days  
Fair value of securities accepted as collateral permitted to sell or repledge $ 2,400.0 $ 2,400.0
Fair value of collateral securities sold or repledged 535.4 233.7
Outstanding commitments to originate loans 146.7 160.2
Letters of credit outstanding 26.3 82.5
Unused Lines Of Credit [Member]    
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]    
Unused lines of credit to commercial and consumer borrowers $ 919.5 $ 590.5
Standby Letters of Credit [Member] | Maximum [Member]    
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]    
Letters of credit, expiration period 1 year