Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Loans, principally due to allowance for credit losses	$ 30.7	$ 36.5
Loan discount	1.6	4.0
Investment securities, unrealized losses	4.6	0.0
Deferred compensation	19.3	19.5
Non-performing loan interest	1.0	1.2
Net operating loss carryforwards	1.7	2.0
Lease liabilities	8.7	9.5
Other	3.5	3.2
Deferred tax assets	71.1	75.9
Deferred tax liabilities:
Fixed assets, principally differences in bases and depreciation	(7.7)	(10.9)
Deferred loan costs	(1.9)	(4.4)
Deferred Tax Liabilities, Unrealized Gains on Trading Securities	0.0
Deferred Tax Liabilities, Unrealized Gains on Available For Sale Securities		(19.2)
Deferred Tax Liabilities, Derivatives	1.0	0.0
Investment in joint venture partnership, principally due to differences in depreciation of partnership assets	(0.9)	(0.9)
Right of use assets	(8.3)	(9.2)
Prepaid amounts	(0.7)	(0.5)
Government agency stock dividends	(1.2)	(1.2)
Goodwill and core deposit intangibles	(51.2)	(50.3)
Mortgage servicing rights	(6.8)	(5.6)
Other	(0.7)	(0.9)
Deferred tax liabilities	(80.4)	(103.1)
Deferred Tax Liabilities, Net	9.3	27.2
Operating Loss Carryforwards [Line Items]
Deferred Income Tax Expense (Benefit)	5.0	$ (6.6)	$ 5.4
Tax Credit Carryforward, Amount	$ 3.1