Consolidated Statements of Cash Flows - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2021
Cash flows from operating activities:
Net income	$ 85.7	$ 47.1	$ 116.4	$ 141.0
Adjustments to reconcile net income to net cash provided by operating activities:
Provision for (reversal of) credit losses	8.4	0.0	68.0	(5.1)
Net gain on disposal of premises and equipment			(2.3)	(1.9)
Depreciation and amortization			41.4	33.4
Net premium amortization on investment securities			16.7	29.0
Net loss on investment securities transactions			24.4	(0.2)
Realized and unrealized net gains on mortgage banking activities			(8.1)	(19.1)
Net gain on sale of OREO			(0.4)	(0.2)
Write-downs of OREO and other assets pending disposal			0.2	0.0
Gain (Loss) on Extinguishment of Debt			(1.4)	0.0
Mortgage servicing rights recovery			(3.4)	(5.9)
Deferred taxes			(4.8)	4.6
Net increase in cash surrender value of company-owned life insurance			(7.6)	(4.1)
Stock-based compensation expense			8.7	7.5
Originations of mortgage loans held for sale			(369.5)	(596.6)
Proceeds from sales of mortgage loans held for sale			387.2	645.1
Changes in operating assets and liabilities:
Increase in interest receivable			(25.9)	(1.8)
Increase in other assets			(14.9)	(21.8)
Increase in accrued interest payable			1.3	0.8
Increase in accounts payable and accrued expenses			212.6	0.7
Net cash provided by operating activities			438.6	205.4
Purchases of investment securities:
Held-to-maturity			(1,316.0)	(1,069.5)
Available-for-sale			(2,521.2)	(1,891.4)
Proceeds from sales, maturities, and pay-downs of investment securities:
Held-to-maturity			304.8	163.9
Available-for-sale			1,753.6	747.8
Purchases of Federal Home Loan Bank and Federal Reserve Bank stock			115.8	0.0
Proceeds from Federal Home Loan Bank and Federal Reserve Bank stock			53.3	0.0
Extensions of credit to clients, net of repayments			(465.6)	172.5
Recoveries of loans charged-off			10.8	7.2
Proceeds from sale of OREO			2.0	1.1
Proceeds from the sale of Health Savings Accounts			1.4	0.0
Acquisition of bank and bank holding company, net of cash and cash equivalents received			2,006.9	0.0
Payments for (Proceeds from) Productive Assets			(27.2)	(5.5)
Net cash used in investing activities			(313.0)	(1,873.9)
Cash flows from financing activities:
Net (decrease) increase in deposits			(2,072.8)	1,790.3
Net increase (decrease) in securities sold under repurchase agreements			51.6	(83.9)
Proceeds from (Repayments of) Other Debt			625.0		$ 0.0
Repayments of long-term debt			(164.0)	0.0
Advances on long-term debt			14.4	0.0
Payments of Stock Issuance Costs			(0.8)	0.0
Proceeds from issuance of common stock, net			0.1	0.4
Purchase and retirement of common stock			(198.9)	(5.4)
Dividends paid to common stockholders			(133.1)	(76.2)
Net cash (used in) provided by financing activities			(1,878.5)	1,625.2
Net decrease in cash and cash equivalents			(1,752.9)	(43.3)
Cash and cash equivalents at beginning of period			2,344.8	2,276.8	2,276.8
Cash and cash equivalents at end of period	$ 591.9	$ 2,233.5	591.9	2,233.5	$ 2,344.8
Supplemental disclosures of cash flow information:
Cash paid during the period for income taxes			42.2	41.3
Cash paid during the period for interest expense			32.0	12.5
Supplemental disclosures of non-cash investing and financing activities:
Right-of-use assets obtained in exchange for operating lease liabilities			22.9	0.8
Net change in unamortized (losses) gains on available-for-sale securities transferred into held-to-maturity			10.9	0.0
Debt Securities, Available-For-Sale, Transfer, Unrealized Gain (Loss)			463.6	672.2
Transfer of loans from held-for-sale to held for investment			19.8	0.0
Transfer of loans to held-for-sale			12.4	0.0
Transfer of loans to other real estate owned			0.4	0.7
Asset Acquisition, Consideration Transferred, Equity Interest Issued and Issuable			1,723.3	0.0
Capitalization of internally originated mortgage servicing rights			2.3	2.0
Investment securities			2,699.0	0.0
Securities Purchased under Agreements to Resell, Increase			101.1	0.0
Loans held for sale			217.0	0.0
Loans held for investment, net			7,645.5	0.0
Premises and equipment			144.7	0.0
Goodwill			478.4	0.0
Other Intangibles			72.9	0.0
Mortgage Servicing Rights			1.3	0.0
Company-owned life insurance			186.6	0.0
Noncash or Part Noncash Acquisition, Deferred Tax Assets Acquired			63.6	0.0
Noncash or Part Noncash Acquisition, Intangible Assets Acquired, Other Real Estate Owned			15.8	0.0
Other assets			198.9	0.0
Total noncash assets acquired			11,824.8	0.0
Noncash or Part Noncash Acquisition, Liabilities Assumed, Deposits			11,688.0	0.0
Securities sold under repurchase agreements			74.0	0.0
Accounts payable and accrued expenses			111.0	0.0
Long-term debt			159.3	0.0
Noncash or Part Noncash Acquisition, Liabilities Assumed, Trust preferred securities			76.1	0.0
Total liabilities assumed			$ 12,108.4	$ 0.0