Consolidated Statements of Cash Flows - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023
Net Cash Provided by (Used in) Operating Activities [Abstract]
Net income	$ 173.9	$ 196.0
Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract]
Provision for credit losses	34.1	26.8
Gain (Loss) on Disposition of Property Plant Equipment	0.0	(1.7)
Depreciation, Depletion and Amortization	41.4	40.5
Accretion (Amortization) of Discounts and Premiums, Investments	0.9	1.7
Debt and Equity Securities, Gain (Loss)	0.0	23.5
Gain (Loss) on Sale of Loans and Leases	(1.4)	(2.3)
Impairment of Real Estate and Other Assets Pending Disposal	4.2	1.1
Deferred Income Tax Expense (Benefit)	6.6	9.9
Life Insurance, Corporate or Bank Owned, Change in Value	(10.7)	(8.1)
Share-Based Payment Arrangement, Noncash Expense	9.7	8.6
Payments for Origination of Mortgage Loans Held-for-sale	(131.4)	(250.8)
Proceeds from Sale of Mortgage Loans Held-for-sale	130.7	255.9
Increase (Decrease) in Operating Capital [Abstract]
Increase (Decrease) in Accrued Interest Receivable, Net	1.8	(10.9)
Increase (Decrease) in Other Operating Assets	10.9	38.5
Increase (Decrease) in Interest Payable, Net	35.5	49.2
Increase (Decrease) in Accounts Payable and Accrued Liabilities	(46.0)	(19.1)
Net Cash Provided by (Used in) Operating Activities, Total	259.8	358.7
Payments to Acquire Investments [Abstract]
Payments to Acquire Debt Securities, Available-for-Sale	(102.2)	0.0
Proceeds from Sale, Maturity and Collection of Investments [Abstract]
Proceeds from Sale and Maturity of Held-to-Maturity Securities	470.0	174.0
Proceeds from Sale and Maturity of Debt Securities, Available-for-Sale	539.4	1,276.3
Purchases of Federal Home Loan Bank and Federal Reserve Bank stock	92.4	187.3
Proceeds from Federal Home Loan Bank and Federal Reserve Bank stock	160.1	196.4
Proceeds from Life Insurance Policy	2.1	5.2
Payments for (Proceeds from) Other Loans and Leases	226.5	(118.1)
Proceeds from Sale of Wholly Owned Real Estate and Real Estate Acquired in Settlement of Loans	12.9	2.6
Payments for (Proceeds from) Productive Assets	(16.4)	(19.4)
Net Cash Provided by (Used in) Investing Activities, Total	1,200.0	1,329.7
Net Cash Provided by (Used in) Financing Activities [Abstract]
Increase (Decrease) in Deposits	(459.0)	(1,394.1)
Proceeds from (Payments for) in Securities Sold under Agreements to Repurchase	(225.5)	(163.4)
Proceeds from (Repayments of) Other Debt	(773.0)	(260.0)
Repayments of Other Long-Term Debt	(0.1)	(0.1)
Proceeds from Issuance of Unsecured Debt	266.5	0.0
Payments for Repurchase of Common Stock	(1.2)	(1.8)
Payments of Ordinary Dividends, Common Stock	(146.9)	(146.4)
Net Cash Provided by (Used in) Financing Activities, Total	(1,339.2)	(1,965.8)
Cash, Cash Equivalents, Restricted Cash, and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect, Total	120.6	(277.4)
Cash, Cash Equivalents, Restricted Cash, and Restricted Cash Equivalents, Beginning Balance	578.0	870.5
Cash, Cash Equivalents, Restricted Cash, and Restricted Cash Equivalents, Ending Balance	698.6	593.1
Supplemental Cash Flow Information [Abstract]
Income Taxes Paid	12.1	39.0
Interest Paid, Excluding Capitalized Interest, Operating Activities	333.9	235.6
Supplemental Cash Flow Elements [Abstract]
Finance Lease, Right-of-Use Asset, Amortization	1.1	6.4
Net change in unamortized losses on available-for-sale investment securities transferred into held-to-maturity	0.0	23.0
Transfer of loans to held-for-sale	0.0	3.1
Real Estate Owned, Transfer to Real Estate Owned	4.6	2.5
Capitalization Of Internally Originated Mortgage Servicing Rights, Noncash Supplemental Disclosures	0.5	0.9
Gains (Losses) on Sales of Other Real Estate	$ (0.4)	$ (0.1)