Investment Securities (Pledged Securities) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Carrying value of available-for-sale securities pledged to secure public and trust department deposits and repurchase agreements	$ 514.9	$ 522.7
Carrying value of held-to-maturity securities pledged to secure public and trust department deposits and repurchase agreements	17.1	18.3
Carrying value of available-for-sale securities pledged to secure additional borrowing capacity at Federal Home Loan Bank and Federal Reserve Bank	61.8	6.7
Pledged Financial Instruments, Not Separately Reported, Held-To-Maturity Securities for Federal Home Loan Bank and Federal Reserve Bank	$ 17.4	$ 19.9