Consolidated Statements of Cash Flows - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Cash Provided by (Used in) Operating Activity, Including Discontinued Operation [Abstract]
Net income	$ 60.2	$ 50.2
Adjustment to Reconcile Net Income to Cash Provided by (Used in) Operating Activity [Abstract]
Provision for credit losses	6.7	20.0
Gain (Loss) on Disposition of Property Plant Equipment	0.2	1.9
Depreciation, Depletion and Amortization	12.5	10.5
Accretion (Amortization) of Discounts and Premiums, Investments	(1.2)	(0.1)
Gain (Loss) on Sale of Loans and Leases	(0.3)	(0.3)
Deferred Income Tax Expense (Benefit)	7.5	7.7
Life Insurance, Corporate or Bank Owned, Change in Value	(4.2)	(1.3)
Share-Based Payment Arrangement, Noncash Expense	2.3	3.9
Payments for Origination of Mortgage Loans Held-for-sale	(10.5)	(10.9)
Proceeds from Sale of Mortgage Loans Held-for-sale	7.0	11.6
Net gains and write-downs of OREO and other assets pending disposal	(1.3)	0.5
Adjustment to Reconcile Net Income to Cash Provided by (Used in) Operating Activity, Increase (Decrease) in Operating Capital [Abstract]
Increase (Decrease) in Accrued Interest Receivable, Net	1.3	8.9
Increase (Decrease) in Other Operating Assets	8.7	55.0
Increase (Decrease) in Interest Payable, Net	(3.8)	(6.2)
Increase (Decrease) in Accounts Payable and Accrued Liabilities	(30.1)	(72.9)
Net Cash Provided by (Used in) Operating Activities, Total	55.0	78.5
Payments to Acquire Investments [Abstract]
Payments to Acquire Debt Securities, Available-for-Sale	(514.2)	(12.7)
Proceeds from Sale, Maturity and Collection of Investments [Abstract]
Proceeds from Sale and Maturity of Held-to-Maturity Securities	49.7	49.0
Proceeds from Sale and Maturity of Debt Securities, Available-for-Sale	179.5	315.1
Payments for (Proceeds from) Short-Term Investments	0.0	27.4
Proceeds from Life Insurance Policy	1.0	0.1
Payments for (Proceeds from) Other Loans and Leases	475.5	458.5
Proceeds from Sale of Wholly Owned Real Estate and Real Estate Acquired in Settlement of Loans	0.0	0.5
Payments for (Proceeds from) Productive Assets	(5.2)	(4.2)
Net Cash Provided by (Used in) Investing Activities, Total	186.3	833.7
Cash Provided by (Used in) Financing Activity, Including Discontinued Operation [Abstract]
Increase (Decrease) in Deposits	(205.3)	(282.8)
Proceeds from (Payments for) in Securities Sold under Agreements to Repurchase	(3.5)	4.1
Proceeds from (Repayments of) Other Debt	0.0	(607.5)
Repayments of Other Long-Term Debt	(0.1)	0.0
Payments for Repurchase of Common Stock	(87.7)	(3.2)
Payments of Ordinary Dividends, Common Stock	(45.7)	(48.0)
Net Cash Provided by (Used in) Financing Activities, Total	(342.3)	(937.4)
Cash, Cash Equivalents, Restricted Cash, and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect, Total	(101.0)	(25.2)
Cash, Cash Equivalents, Restricted Cash, and Restricted Cash Equivalents, Beginning Balance	1,309.7	896.6
Cash, Cash Equivalents, Restricted Cash, and Restricted Cash Equivalents, Ending Balance	1,208.7	871.4
Supplemental Cash Flow Information [Abstract]
Interest Paid, Excluding Capitalized Interest, Operating Activity	74.4	104.5
OCI, Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Adjustment from AOCI for Amortization of Gain (Loss), before Tax	(0.1)	0.0
Supplemental Cash Flow Elements [Abstract]
Finance Lease, Right-of-Use Asset, Amortization	0.0	7.2
Right-of-Use Asset Obtained in Exchange for Finance Lease Liability	0.4	0.0
Transfer of loans to other real estate owned	1.8	0.1
Transfer of premises and equipment to held-for-sale	1.5	0.0
Capitalization Of Internally Originated Mortgage Servicing Rights, Noncash Supplemental Disclosures	0.2	0.1
Transfer of loans from held-for-sale to held for investment	$ 6.5	$ 0.0