Investment Securities Pledged Securities (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Investments [Abstract]
Carrying value of available-for-sale securities pledged to secure public and trust department deposits and repurchase agreements	$ 517.9	$ 495.5
Carrying value of held-to-maturity securities pledged to secure public and trust department deposits and repurchase agreements	20.0	21.4
Carrying value of available-for-sale securities pledged to secure additional borrowing capacity at FHLB and FRB	9.2	11.1
Carrying value of held-to-maturity securities pledged to secure additional borrowing capacity at FHLB and FRB	$ 22.2	$ 23.3