Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Cash flows from operating activities:
Net income	$ 302.1	$ 226.0	$ 257.5
Provision for Loan, Lease, and Other Losses	26.8	67.8	32.2
Adjustments to reconcile net income from operations to net cash provided by operating activities:
Net loss (gain) on disposal of premises and equipment	8.4	(2.9)	0.7
Depreciation and amortization	50.8	57.0	53.8
Net (discount) premium amortization on investment securities	(0.2)	1.1	2.4
Net (gain) loss on investment securities transactions	0.0	0.0	23.5
Realized and unrealized net gains on mortgage banking activities	(1.5)	(1.8)	(2.8)
Net gains and write-downs of OREO and other assets pending disposal	0.5	3.9	1.0
Gain (Loss) on Extinguishment of Debt	(0.1)	0.0	0.0
Valuation allowance for loans held-for-sale	6.0	0.0	0.0
Deferred Income Tax Expense (Benefit)	11.3	21.3	20.4
Life Insurance, Corporate or Bank Owned, Change in Value	(11.1)	(14.7)	(9.7)
Stock-based compensation expense	13.3	11.8	4.7
Payment for Origination, Loan, Mortgage, Held-for-Sale	(66.7)	(149.6)	(351.5)
Proceeds from Sale, Loan, Mortgage, Held-for-Sale	67.6	150.3	359.5
Gain on sale of consumer credit card loans	(4.3)	0.0	0.0
Gain on sale of branches, net	(62.7)	0.0	0.0
Changes in operating assets and liabilities:
Decrease (increase) in accrued interest receivable	13.4	12.3	(10.8)
Decrease (increase) in other assets	73.0	(23.2)	80.5
(Decrease) increase in accrued interest payable	(9.5)	(5.2)	37.7
(Decrease) increase in accounts payable and accrued expenses	(111.5)	0.9	(71.1)
Net cash provided by operating activities	305.6	355.0	428.0
Purchases of investment securities:
Payments to Acquire Debt Securities, Available-for-Sale	(1,437.0)	(102.2)	(134.7)
Proceeds from maturities, pay-downs, calls and sales of investment securities:
Held-to-maturity	349.7	524.7	227.3
Available-for-sale	1,390.1	924.2	1,390.7
Payments for (Proceeds from) Short-Term Investments	72.5	45.8	(24.6)
Proceeds from company-owned life insurance settlements	1.1	4.1	5.2
Proceeds from sales of consumer credit card loans	81.3	0.0	0.0
Net change in loans held for investment	2,159.9	372.3	(185.8)
Proceeds from sale of OREO	0.9	12.9	3.5
Net cash distributed in sale of branches	(279.1)	0.0	0.0
Capital expenditures, net of sales	(27.8)	(23.0)	(28.2)
Net cash provided by investing activities	2,311.6	1,758.8	1,253.4
Net increase (decrease) in cash and cash equivalents	413.1	318.6	(292.5)
Cash and cash equivalents at beginning of period	896.6	578.0	870.5
Cash and cash equivalents at end of period	1,309.7	896.6	578.0
Finite-Lived Intangible Assets, Gross		177.5	177.5
Cash flows from financing activities:
Net decrease in deposits	(285.7)	(307.5)	(1,750.5)
Net decrease in securities sold under repurchase agreements	(44.3)	(258.8)	(270.2)
Proceeds from (Repayments of) Other Debt	(1,567.5)	(1,285.5)	276.0
Repayments of Other Long-term Debt	112.6	0.2	0.1
Proceeds on long-term debt	0.0	253.9	0.0
Payments of Debt Issuance Costs	(2.8)	0.0	0.0
Proceeds from Issuance of Subordinated Long-Term Debt	125.0	0.0	0.0
Purchase and retirement of common stock	(121.9)	(1.2)	(34.0)
Dividends paid to common stockholders	(194.3)	(195.9)	(195.1)
Net cash used in financing activities	(2,204.1)	(1,795.2)	(1,973.9)
Supplemental disclosures of noncash investing and financing activities:
Transfer of held-to-maturity to available-for-sale securities	0.0	0.0	23.0
Transfer of Loans Held-for-sale to Portfolio Loans	0.0	0.0	29.6
Amortization of unrealized gains and losses on transfers of securities	370.0	0.0	30.4
Transfer of premises and equipment held-for-sale	19.7	0.0	0.0
Right-of-use assets obtained in exchange for operating lease liabilities	6.8	1.6	6.4
Transfer of loans to other real estate owned	0.5	4.6	8.3
Capitalization Of Internally Originated Mortgage Servicing Rights, Noncash Supplemental Disclosures	0.5	0.7	1.1
Supplemental disclosures of cash flow information:
Cash paid during the period for income taxes, net(1)	(36.6)	(34.3)	(51.2)
Cash paid during the period for interest expense	362.4	475.7	363.6
Valuation allowance for loans held-for-sale	6.0	0.0	0.0
Common Stock Including Additional Paid in Capital [Member]
Cash flows from operating activities:
Net income	$ 0.0	$ 0.0	$ 0.0