Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Deferred tax assets:
Loans, principally due to allowance for credit losses	$ 50.7	$ 56.7
Loan discount	11.5	21.7
Investment securities, unrealized losses	106.1	118.1
Deferred compensation	22.5	18.6
Non-performing loan interest	2.5	2.2
Net operating loss carryforwards	1.0	1.1
Lease liabilities	8.9	11.0
Other	4.3	4.8
Deferred tax assets	228.7	264.2
Deferred tax liabilities:
Fixed assets, principally differences in bases and depreciation	(20.6)	(19.8)
Deferred loan costs	(3.2)	(4.1)
Investment in joint venture partnership, principally due to differences in depreciation of partnership assets	(0.8)	(2.4)
Right of use assets	(8.2)	(10.3)
Prepaid amounts	(0.8)	(0.9)
Government agency stock dividends	(1.2)	(1.3)
Goodwill and other intangibles	(68.6)	(68.1)
Mortgage servicing rights	(6.4)	(6.9)
Other	(0.5)	(0.4)
Deferred tax liabilities	(110.3)	(114.2)
Deferred Tax Liabilities, Net	118.4	150.0
Operating Loss Carryforwards [Line Items]
Deferred Income Tax Expense (Benefit)	21.3	20.4	$ (4.7)
Other real estate owned write-downs and carrying costs	0.0	1.4
Deferred Tax Assets, Derivative Instruments	0.8	0.3
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Reserves	7.2	10.7
Deferred Tax Assets, Contract Incentives	6.4	7.5
Deferred Tax Assets, Discount on Acquired Investment Securities	6.8	$ 10.1
Federal
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards	2.0
State
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards	$ 11.6