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Investment Securities - Proceeds from Calls and Sales, Realized Gains and Losses and Interest Income (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Proceeds from calls and sales of investment securities      
Proceeds from calls of available for sale securities $ 0.0 $ 121.2 $ 25.0
Proceeds from sales of available for sale securities 0.0 825.4 2,500.0
Available-for-sale Securities, Gross Realized Gain (Loss), Disclosures [Abstract]      
Gross realized gains on sales of investment securities 0.0 27.4 18.8
Gross realized losses on sales of investment securities 0.0 0.1 6.5
Income tax expense related to net realized gains on sale of investment securities 0.0 10.8 4.9
Interest income from taxable and nontaxable investment securities      
Taxable interest income 102.3 83.0 73.6
Non-taxable interest income $ 0.1 $ 0.0 $ 0.0