Investments in Qualified Affordable Housing Projects, Federal Historic Projects and New Market Tax Credits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Variable Interest Entity [Line Items]
Expense related to qualified affordable housing projects	$ 4.2	$ 3.4	$ 2.6
Remaining investment in qualified affordable housing projects	84.9	51.4
Equity method investments	13.3	17.3
Remaining unfunded equity contributions	72.0	48.1
Impairment on tax credit projects	12.9	7.9	$ 0.0
Other noninterest expense
Variable Interest Entity [Line Items]
Impairment on tax credit projects	9.7
Operating expense
Variable Interest Entity [Line Items]
Impairment on tax credit projects	$ 12.3	9.3
Other Operating Expense
Variable Interest Entity [Line Items]
Impairment on tax credit projects		$ 6.0