Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Cash flows from operating activities:
Net income	$ 257.5	$ 202.2	$ 192.1
Adjustments to reconcile net income from operations to net cash provided by operating activities:
Provision for (reversal of) credit losses	32.2	82.7	(14.6)
Net (gain) loss on disposal of premises and equipment	0.7	(2.0)	(1.8)
Depreciation and amortization	53.8	55.5	44.4
Net premium amortization on investment securities	2.4	17.0	38.8
Net loss (gain) on investment securities transactions	23.5	24.4	(1.1)
Realized and unrealized net gains on mortgage banking activities	(2.8)	(9.4)	(26.2)
Net gain on sale of OREO	(0.1)	(0.7)	(0.3)
Write-downs of OREO and other assets pending disposal	1.1	2.8	0.0
Gain (Loss) on Extinguishment of Debt	0.0	(1.4)	0.0
Mortgage servicing rights impairment (recovery)	0.0	(3.4)	(6.9)
Deferred taxes	20.4	(4.7)	5.0
Net increase in cash surrender value of company-owned life insurance	(9.7)	(11.1)	(6.1)
Stock compensation expense	4.7	9.6	8.9
Originations of mortgage loans held for sale	(351.5)	(429.5)	(817.4)
Proceeds from sales of mortgage loans held for sale	359.5	461.6	883.9
Changes in operating assets and liabilities:
(Increase) decrease in interest receivable	(10.8)	(37.8)	3.7
Decrease (increase) in other assets	80.5	(12.9)	(16.4)
Increase (decrease) in accrued interest payable	37.7	8.4	(2.0)
(Decrease) increase in accounts payable and accrued expenses	(71.1)	183.1	(1.7)
Net cash provided by operating activities	428.0	534.4	282.3
Purchases of investment securities:
Held-to-maturity	(0.5)	(858.5)	(1,238.0)
Available-for-sale	(134.2)	(3,309.4)	(2,717.8)
Proceeds from maturities, pay-downs, calls and sales of investment securities:
Held-to-maturity	227.3	370.5	257.6
Available-for-sale	1,390.7	1,926.5	1,118.0
Purchases of Federal Home Loan Bank and Federal Reserve Bank Stock	(290.8)	(241.4)	0.0
Proceeds from Federal Home Loan Bank and Federal Reserve Bank Stock	266.2	112.2	0.0
Proceeds from bank-owned life insurance settlements	5.2	1.3	1.0
Extensions of credit to clients, net of repayments	(185.8)	(951.4)	467.8
Proceeds from sales of OREO	3.5	3.4	1.7
Proceeds from the sale of Health Savings Accounts	0.0	1.4	0.0
Acquisition of bank and bank holding company, net of cash and cash equivalents received	0.0	2,006.9	0.0
Capital expenditures, net of sales	(28.2)	(10.5)	(10.3)
Net cash provided by (used in) investing activities	1,253.4	(949.0)	(2,120.0)
Cash flows from financing activities:
Net (decrease) increase in deposits	(1,750.5)	(2,884.0)	2,052.6
Net (decrease) increase in securities sold under repurchase agreements	(270.2)	28.9	(40.3)
Proceeds from (Repayments of) Other Debt	276.0	2,327.0	0.0
Repayments of Other Long-term Debt	0.1	164.1	0.0
Advances on long-term debt	0.0	14.3	0.0
Payments of Stock Issuance Costs	0.0	(0.8)	0.0
Proceeds from issuance of common stock	0.0	0.1	0.4
Purchase and retirement of common stock	(34.0)	(199.0)	(5.4)
Dividends paid to common stockholders	(195.1)	(182.1)	(101.6)
Net cash used in financing activities	(1,973.9)	(1,059.7)	1,905.7
Net (decrease) increase in cash and cash equivalents	(292.5)	(1,474.3)	68.0
Cash and cash equivalents at beginning of period	870.5	2,344.8	2,276.8
Cash and cash equivalents at end of period	578.0	870.5	2,344.8
Supplemental disclosures of cash flow information:
Cash paid during the period for income taxes	(51.2)	(57.8)	(56.8)
Cash paid during the period for interest expense	363.6	32.0	19.3
Supplemental disclosures of noncash investing and financing activities:
Transfer of held-to-maturity to available-for-sale securities	23.0	10.9	0.0
Non-cash transfer of securities from available-for-sale to held-to-maturity	0.0	463.6	672.2
Transfer of Loans Held-for-sale to Portfolio Loans	29.6	19.8	0.0
Amortization of unrealized gains and losses on transfers of securities	30.4	12.4	0.0
Right-of-use assets obtained in exchange for operating lease liabilities	6.4	22.9	5.9
Transfer of loans to other real estate owned	8.3	0.4	0.9
Asset Acquisition, Consideration Transferred, Equity Interest Issued and Issuable	0.0	1,723.3	0.0
Capitalization of internally originated mortgage servicing rights	1.1	2.5	3.7
Supplemental schedule of noncash investing activities from acquisitions:
Investment securities	0.0	2,699.0	0.0
Loans held for sale	0.0	217.0	0.0
Loans held for investment, net	0.0	7,645.5	0.0
Premises and equipment	0.0	144.7	0.0
Goodwill	0.0	479.3	0.0
Other intangibles	0.0	72.9	0.0
Mortgage servicing rights	0.0	1.3	0.0
Company-owned life insurance	0.0	186.6	0.0
Deferred tax assets	0.0	60.2	0.0
Other real estate owned	0.0	15.8	0.0
Other assets	0.0	200.8	0.0
Total noncash assets acquired	0.0	11,824.2	0.0
Deposits	0.0	11,688.0	0.0
Securities sold under repurchase agreements	0.0	74.0	0.0
Accounts payable and accrued expenses	0.0	110.4	0.0
Long-term debt	0.0	159.3	0.0
Subordinated debentures held by subsidiary trusts	0.0	76.1	0.0
Total liabilities assumed	0.0	(12,107.8)	0.0
Securities Purchased under Agreements to Resell, Increase	$ 0.0	101.1	0.0
Finite-Lived Intangible Assets, Gross		$ 177.5	$ 106.0