Other Assets - Affordable Housing and Other Tax Credit Investment Partnerships (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Affordable housing and other tax credit investment partnership interests
Affordable housing tax credits and other benefits recognized	$ 4.5	$ 4.8	$ 4.7
Provision for income taxes
Affordable housing and other tax credit investment partnership interests
Amortization	3.5	3.5	$ 3.3
Other assets
Affordable housing and other tax credit investment partnership interests
Investment partnership interest	$ 52.3	$ 12.9