Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Deferred tax assets:
Loans, principally due to allowance for credit losses	$ 56.7	$ 54.8
Loan discount	21.7	32.5
Investment securities, unrealized losses	118.1	157.9
Deferred compensation	18.6	26.8
Non-performing loan interest	2.2	2.6
Net operating loss carryforwards	1.1	1.5
Lease liabilities	11.0	12.0
Other	4.8	2.4
Deferred tax assets	264.2	323.2
Deferred tax liabilities:
Fixed assets, principally differences in bases and depreciation	(19.8)	(17.3)
Deferred loan costs	(4.1)	(3.3)
Investment in joint venture partnership, principally due to differences in depreciation of partnership assets	(2.4)	(1.3)
Right of use assets	(10.3)	(11.3)
Prepaid amounts	(0.9)	(0.9)
Government agency stock dividends	(1.3)	(1.2)
Goodwill and other intangibles	(68.1)	(68.2)
Mortgage servicing rights	(6.9)	(7.4)
Other	(0.4)	(1.8)
Deferred tax liabilities	(114.2)	(112.7)
Deferred Tax Liabilities, Net	150.0	210.5
Operating Loss Carryforwards [Line Items]
Deferred Income Tax Expense (Benefit)	20.4	(4.7)	$ 5.0
Tax Credit Carryforward, Amount	2.4
Other real estate owned write-downs and carrying costs	1.4	1.5
Deferred Tax Assets, Derivative Instruments	0.3	0.5
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Reserves	10.7	8.8
Deferred Tax Assets, Contract Incentives	7.5	8.5
Deferred Tax Assets, Discount on Acquired Investment Securities	$ 10.1	$ 13.4