Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Deferred tax assets:
Loans, principally due to allowance for credit losses	$ 47.7	$ 50.7
Loan discount	7.0	11.5
Investment securities, unrealized losses	59.3	106.1
Deferred compensation	20.3	22.5
Non-performing loan interest	3.2	2.5
Net operating loss carryforwards	0.6	1.0
Lease liabilities	8.1	8.9
Other	3.4	4.3
Deferred tax assets	164.5	228.7
Deferred tax liabilities:
Fixed assets, principally differences in bases and depreciation	(17.3)	(20.6)
Deferred loan costs	(2.8)	(3.2)
Investment in joint venture partnership, principally due to differences in depreciation of partnership assets	(1.6)	(0.8)
Right of use assets	(7.5)	(8.2)
Prepaid amounts	(0.6)	(0.8)
Government agency stock dividends	(1.2)	(1.2)
Goodwill and other intangibles	(67.6)	(68.6)
Mortgage servicing rights	(5.8)	(6.4)
Other	(0.4)	(0.5)
Deferred tax liabilities	(104.9)	(110.3)
Deferred Tax Liabilities, Net	59.6	118.4
Operating Loss Carryforwards [Line Items]
Deferred Income Tax Expense (Benefit)	11.3	21.3	$ 20.4
Deferred Tax Assets, Derivative Instruments	0.0	0.8
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Reserves	6.0	7.2
Deferred Tax Assets, Contract Incentives	5.3	6.4
Deferred Tax Assets, Discount on Acquired Investment Securities	3.6	6.8
Deferred Tax Liabilities, Derivatives	0.1	$ 0.0
Federal
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards	$ 1.7