Consolidated Statements of Cash Flows - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Net Cash Provided by (Used in) Operating Activities [Abstract]
Net income	$ 56.3	$ (33.4)
Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract]
Provision for credit losses	15.2	61.3
Gain (Loss) on Disposition of Property Plant Equipment	0.1	(0.2)
Depreciation, Depletion and Amortization	13.7	13.3
Accretion (Amortization) of Discounts and Premiums, Investments	(0.6)	8.1
Debt and Equity Securities, Gain (Loss)	23.4	0.1
Gain (Loss) on Sale of Loans and Leases	(0.8)	(3.3)
Impairment of Real Estate and Other Assets Pending Disposal	0.1	0.0
Gain (Loss) on Extinguishment of Debt	0.0	(1.4)
Mortgage Servicing Rights (MSR) Impairment (Recovery)	0.0	(3.4)
Deferred Income Tax Expense (Benefit)	11.3	(6.5)
Life Insurance, Corporate or Bank Owned, Change in Value	(3.4)	(2.0)
Share-Based Payment Arrangement, Noncash Expense	2.3	2.4
Payments for Origination of Mortgage Loans Held-for-sale	(80.4)	(98.8)
Proceeds from Sale of Mortgage Loans Held-for-sale	79.6	112.0
Increase (Decrease) in Operating Capital [Abstract]
Increase (Decrease) in Accrued Interest Receivable, Net	4.6	(0.6)
Increase (Decrease) in Other Operating Assets	7.7	4.3
Increase (Decrease) in Interest Payable, Net	11.6	(0.1)
Increase (Decrease) in Accounts Payable and Accrued Liabilities	(83.0)	13.1
Net Cash Provided by (Used in) Operating Activities, Total	57.7	64.9
Payments to Acquire Investments [Abstract]
Payments to Acquire Held-to-Maturity Securities	0.0	(370.0)
Payments to Acquire Debt Securities, Available-for-Sale	0.0	(1,144.5)
Proceeds from Sale, Maturity and Collection of Investments [Abstract]
Proceeds from Sale and Maturity of Held-to-Maturity Securities	54.9	106.4
Proceeds from Sale and Maturity of Debt Securities, Available-for-Sale	987.3	826.5
Purchases of Federal Home Loan Bank and Federal Reserve Bank stock	73.5	0.0
Proceeds from Federal Home Loan Bank and Federal Reserve Bank stock	57.5	0.0
Proceeds from Life Insurance Policy	1.9	0.0
Payments for (Proceeds from) Other Loans and Leases	(152.8)	106.4
Proceeds from Sale of Wholly Owned Real Estate and Real Estate Acquired in Settlement of Loans	0.0	0.3
Cash Acquired from Acquisition	0.0	2,006.9
Payments for (Proceeds from) Productive Assets	(7.0)	(9.5)
Net Cash Provided by (Used in) Investing Activities, Total	868.3	1,522.5
Net Cash Provided by (Used in) Financing Activities [Abstract]
Increase (Decrease) in Deposits	(966.6)	130.8
Proceeds from (Payments for) in Securities Sold under Agreements to Repurchase	(82.1)	(55.0)
Proceeds from (Repayments of) Other Debt	383.0	0.0
Repayments of Other Long-Term Debt	0.0	(157.9)
Proceeds from Issuance of Unsecured Debt	0.0	8.0
Payments of Stock Issuance Costs	0.0	(0.8)
Proceeds from Issuance of Common Stock	0.0	0.1
Payments for Repurchase of Common Stock	(1.8)	(1.4)
Payments of Ordinary Dividends, Common Stock	(48.3)	(44.7)
Net Cash Provided by (Used in) Financing Activities, Total	(715.8)	(120.9)
Cash, Cash Equivalents, Restricted Cash, and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect, Total	210.2	1,466.5
Cash, Cash Equivalents, Restricted Cash, and Restricted Cash Equivalents, Beginning Balance	870.5	2,344.8
Cash, Cash Equivalents, Restricted Cash, and Restricted Cash Equivalents, Ending Balance	1,080.7	3,811.3
Supplemental Cash Flow Information [Abstract]
Income Taxes Paid	0.0	0.1
Interest Paid, Excluding Capitalized Interest, Operating Activities	65.4	6.0
Supplemental Cash Flow Elements [Abstract]
Asset Acquisition, Consideration Transferred, Equity Interest Issued and Issuable	0.0	1,723.3
Finance Lease, Right-of-Use Asset, Amortization	0.6	19.9
Net change in unamortized losses on available-for-sale investment securities transferred into held-to-maturity	23.0	10.9
Debt Securities, Available-For-Sale, Transfer, Unrealized Gain (Loss)	0.0	463.6
Transfer of loans to held-for-sale	3.1	0.0
Transfer of loans from held-for-sale to held for investment	0.0	19.8
Real Estate Owned, Transfer to Real Estate Owned	$ 0.8	$ 0.0