Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Loans, principally due to allowance for credit losses	$ 36.5	$ 18.1
Loan discount	4.0	7.2
Deferred compensation	19.5	18.1
Non-performing loan interest	1.2	1.0
Other real estate owned write-downs and carrying costs	0.0	0.1
Tax credit carryforwards	0.0	0.2
Net operating loss carryforwards	2.0	2.6
Lease liabilities	9.5	9.9
Other	3.2	0.9
Deferred tax assets	75.9	58.1
Deferred tax liabilities:
Fixed assets, principally differences in bases and depreciation	(10.9)	(8.8)
Deferred loan costs	(4.4)	(2.8)
Deferred Tax Liabilities, Unrealized Gains on Trading Securities	19.2
Deferred Tax Liabilities, Unrealized Gains on Available For Sale Securities		(3.9)
Investment in joint venture partnership, principally due to differences in depreciation of partnership assets	(0.9)	(0.7)
Right of use assets	(9.2)	(9.7)
Prepaid amounts	(0.5)	(0.5)
Government agency stock dividends	(1.2)	(1.2)
Goodwill and core deposit intangibles	(50.3)	(49.2)
Mortgage servicing rights	(5.6)	(7.2)
Other	(0.9)	(0.8)
Deferred tax liabilities	(103.1)	(84.8)
Deferred Tax Liabilities, Net	27.2	26.7
Operating Loss Carryforwards [Line Items]
Deferred Income Tax Expense (Benefit)	(6.6)	$ 5.4	$ 15.8
Tax Credit Carryforward, Amount	$ 3.4