Investment Securities (Pledged Securities) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Carrying value of available-for-sale securities pledged to secure public and trust department deposits and repurchase agreements	$ 543.1	$ 517.9
Carrying value of held-to-maturity securities pledged to secure public and trust department deposits and repurchase agreements	19.0	20.0
Carrying value of available-for-sale securities pledged to secure additional borrowing capacity at Federal Home Loan Bank and Federal Reserve Bank	7.9	9.2
Pledged Financial Instruments, Not Separately Reported, Held-To-Maturity Securities for Federal Home Loan Bank and Federal Reserve Bank	$ 20.9	$ 22.2