Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Cash flows from operating activities:
Net income	$ 226.0	$ 257.5	$ 202.2
Adjustments to reconcile net income from operations to net cash provided by operating activities:
Provision for credit losses	67.8	32.2	82.7
Net gain (loss) on disposal of premises and equipment	(2.9)	0.7	(2.0)
Depreciation and amortization	57.0	53.8	55.5
Net premium amortization on investment securities	1.1	2.4	17.0
Net loss on investment securities transactions	0.0	23.5	24.4
Realized and unrealized net gains on mortgage banking activities	(1.8)	(2.8)	(9.4)
Gain (Loss) on Extinguishment of Debt	0.0	0.0	(1.4)
Mortgage servicing rights impairment (recovery)	0.0	0.0	(3.4)
Deferred Income Tax Expense (Benefit)	21.3	20.4	(4.7)
Life Insurance, Corporate or Bank Owned, Change in Value	(14.7)	(9.7)	(11.1)
Stock-based compensation expense	11.8	4.7	9.6
Payment for Origination, Loan, Mortgage, Held-for-Sale	(149.6)	(351.5)	(429.5)
Proceeds from Sale, Loan, Mortgage, Held-for-Sale	150.3	359.5	461.6
Net gains and write-downs of OREO and other assets pending disposal	3.9	1.0	2.1
Changes in operating assets and liabilities:
Decrease (increase) in accrued interest receivable	12.3	(10.8)	(37.8)
(Increase) decrease in other assets	(23.2)	80.5	(12.9)
(Decrease) increase in accrued interest payable	(5.2)	37.7	8.4
Increase (decrease) in accounts payable and accrued expenses	0.9	(71.1)	183.1
Net cash provided by operating activities	355.0	428.0	534.4
Purchases of investment securities:
Payments to Acquire Debt Securities, Available-for-Sale	(102.2)	(134.7)	(4,167.9)
Proceeds from maturities, pay-downs, calls and sales of investment securities:
Held-to-maturity	524.7	227.3	370.5
Available-for-sale	924.2	1,390.7	1,926.5
Payments for (Proceeds from) Short-Term Investments	45.8	(24.6)	(129.2)
Proceeds from company-owned life insurance settlements	4.1	5.2	1.3
Net change in loans held for investment	372.3	(185.8)	(951.4)
Proceeds from sale of OREO	12.9	3.5	3.4
Proceeds from the sale of Health Savings Accounts	0.0	0.0	1.4
Acquisition of bank and bank holding company, net of cash and cash equivalents received	0.0	0.0	2,006.9
Capital expenditures, net of sales	(23.0)	(28.2)	(10.5)
Net cash provided by (used in) investing activities	1,758.8	1,253.4	(949.0)
Net increase (decrease) in cash and cash equivalents	318.6	(292.5)	(1,474.3)
Cash and cash equivalents at beginning of period	578.0	870.5	2,344.8
Cash and cash equivalents at end of period	896.6	578.0	870.5
Securities Purchased under Agreements to Resell, Increase	0.0	0.0	101.1
Finite-Lived Intangible Assets, Gross		177.5	177.5
Cash flows from financing activities:
Net decrease in deposits	(307.5)	(1,750.5)	(2,884.0)
Net (decrease) increase in securities sold under repurchase agreements	(258.8)	(270.2)	28.9
Proceeds from (Repayments of) Other Debt	(1,285.5)	276.0	2,327.0
Repayments of Other Long-term Debt	0.2	0.1	164.1
Advances on long-term debt	253.9	0.0	14.3
Payments of Stock Issuance Costs	0.0	0.0	(0.8)
Proceeds from issuance of common stock	0.0	0.0	0.1
Purchase and retirement of common stock	(1.2)	(34.0)	(199.0)
Dividends paid to common stockholders	(195.9)	(195.1)	(182.1)
Net cash used in financing activities	(1,795.2)	(1,973.9)	(1,059.7)
Supplemental schedule of noncash investing activities from acquisitions:
Investment securities	0.0	0.0	2,699.0
Loans held for sale	0.0	0.0	217.0
Loans held for investment, net	0.0	0.0	7,645.5
Premises and equipment	0.0	0.0	144.7
Goodwill	0.0	0.0	479.3
Other intangibles	0.0	0.0	72.9
Mortgage servicing rights	0.0	0.0	1.3
Company-owned life insurance	0.0	0.0	186.6
Deferred tax assets	0.0	0.0	60.2
Other real estate owned	0.0	0.0	15.8
Other assets	0.0	0.0	200.8
Total noncash assets acquired	0.0	0.0	11,824.2
Deposits	0.0	0.0	11,688.0
Securities sold under repurchase agreements	0.0	0.0	74.0
Accounts payable and accrued expenses	0.0	0.0	110.4
Long-term debt	0.0	0.0	159.3
Subordinated debentures held by subsidiary trusts	0.0	0.0	76.1
Total liabilities assumed	0.0	0.0	(12,107.8)
Supplemental disclosures of noncash investing and financing activities:
Transfer of held-to-maturity to available-for-sale securities	0.0	23.0	10.9
Non-cash transfer of securities from available-for-sale to held-to-maturity	0.0	0.0	463.6
Transfer of Loans Held-for-sale to Portfolio Loans	0.0	29.6	19.8
Amortization of unrealized gains and losses on transfers of securities	0.0	30.4	12.4
Right-of-use assets obtained in exchange for operating lease liabilities	1.6	6.4	22.9
Transfer of loans to other real estate owned	4.6	8.3	0.4
Asset Acquisition, Consideration Transferred, Equity Interest Issued and Issuable	0.0	0.0	1,723.3
Supplemental disclosures of cash flow information:
Cash paid during the period for income taxes	(34.3)	(51.2)	(57.8)
Cash paid during the period for interest expense	475.7	363.6	32.0
Common Stock Including Additional Paid in Capital [Member]
Cash flows from operating activities:
Net income	$ 0.0	$ 0.0	$ 0.0