Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Loans, principally due to allowance for loan losses	$ 18.1	$ 18.4
Loan discount	7.2	8.3
Investment securities, unrealized losses	0.0	8.9
Deferred compensation	18.1	17.0
Non-performing loan interest	1.0	1.2
Other real estate owned write-downs and carrying costs	0.1	0.3
Tax credit carryforwards	0.2	0.1
Net operating loss carryforwards	2.6	3.9
Lease liabilities	9.9	0.0
Other	0.9	0.1
Deferred tax assets	58.1	58.2
Deferred tax liabilities:
Fixed assets, principally differences in bases and depreciation	(8.8)	(6.9)
Deferred loan costs	(2.8)	(2.6)
Deferred Tax Liabilities, Unrealized Gains on Trading Securities	3.9
Deferred Tax Liabilities, Unrealized Gains on Available For Sale Securities		0.0
Investment in joint venture partnership, principally due to differences in depreciation of partnership assets	(0.7)	(0.8)
Right of use assets	(9.7)	0.0
Prepaid amounts	(0.5)	(0.6)
Government agency stock dividends	(1.2)	(1.5)
Goodwill and core deposit intangibles	(49.2)	(44.2)
Mortgage servicing rights	(7.2)	(6.4)
Other	(0.8)	(3.8)
Deferred tax liabilities	(84.8)	(66.8)
Deferred Tax Liabilities, Net	26.7	8.6
Operating Loss Carryforwards [Line Items]
Deferred Income Tax Expense (Benefit)	$ 5.4	$ 15.8	$ 20.9