Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Deferred tax assets:
Loans, principally due to allowance for credit losses	$ 54.8	$ 30.7
Loan discount	32.5	1.6
Investment securities, unrealized losses	157.9	4.6
Deferred compensation	26.8	19.3
Non-performing loan interest	2.6	1.0
Net operating loss carryforwards	1.5	1.7
Lease liabilities	12.0	8.7
Other	2.4	3.5
Deferred tax assets	323.2	71.1
Deferred tax liabilities:
Fixed assets, principally differences in bases and depreciation	(17.3)	(7.7)
Deferred loan costs	(3.3)	(1.9)
Deferred Tax Liabilities, Derivatives	0.0	1.0
Investment in joint venture partnership, principally due to differences in depreciation of partnership assets	(1.3)	(0.9)
Right of use assets	(11.3)	(8.3)
Prepaid amounts	(0.9)	(0.7)
Government agency stock dividends	(1.2)	(1.2)
Goodwill and other intangibles	(68.2)	(51.2)
Mortgage servicing rights	(7.4)	(6.8)
Other	(1.8)	(0.7)
Deferred tax liabilities	(112.7)	(80.4)
Deferred Tax Liabilities, Net	210.5	9.3
Operating Loss Carryforwards [Line Items]
Deferred Income Tax Expense (Benefit)	(4.7)	5.0	$ (6.6)
Tax Credit Carryforward, Amount	2.7
Other real estate owned write-downs and carrying costs	1.5	0.0
Deferred Tax Assets, Derivative Instruments	0.5	0.0
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Reserves	8.8	0.0
Deferred Tax Assets, Contract Incentives	8.5	0.0
Deferred Tax Assets, Discount on Acquired Investment Securities	$ 13.4	$ 0.0