Investment Securities Pledged Securities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Investments [Abstract]
Carrying value of available-for-sale securities pledged to secure public and trust department deposits and repurchase agreements	$ 522.7	$ 517.9
Carrying value of held-to-maturity securities pledged to secure public and trust department deposits and repurchase agreements	18.3	20.0
Carrying value of available-for-sale securities pledged to secure additional borrowing capacity at FHLB and FRB	6.7	9.2
Carrying value of held-to-maturity securities pledged to secure additional borrowing capacity at FHLB and FRB	$ 19.9	$ 22.2