Investments in Qualified Affordable Housing Projects, Federal Historic Projects and New Market Tax Credits (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Variable Interest Entity [Line Items]
Expense related to qualified affordable housing projects	$ 1.0	$ 0.8	$ 3.1	$ 2.3
Remaining investment in qualified affordable housing projects	52.2		52.2		$ 51.4
Equity method investments	11.0		11.0		17.3
Remaining unfunded equity contributions	42.3		42.3		$ 48.1
Investment tax credit	3.2	2.3	6.6	2.3
Impairment on tax credit projects	3.2	3.1	6.5	3.1
Other Noninterest Expense [Member]
Variable Interest Entity [Line Items]
Impairment on tax credit projects	$ 2.5	$ 2.0	$ 5.1	$ 2.0