Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Cash flows from operating activities:
Net income	$ 202.2	$ 192.1	$ 161.2
Adjustments to reconcile net income from operations to net cash provided by operating activities:
Provision for (reversal of) credit losses	82.7	(14.6)	56.9
Net (gain) loss on disposal of premises and equipment	(2.0)	(1.8)	0.3
Depreciation and amortization	55.5	44.4	45.1
Net premium amortization on investment securities	17.0	38.8	15.9
Net loss (gain) on investment securities transactions	24.4	(1.1)	(0.3)
Realized and unrealized net gains on mortgage banking activities	(9.4)	(26.2)	(49.3)
Net gain on sale of investments in unrelated entities	0.0	0.0	(1.0)
Net gain on sale of OREO	(0.7)	(0.3)	(0.9)
Write-downs of OREO and other assets pending disposal	2.8	0.0	0.1
Gain (Loss) on Extinguishment of Debt	(1.4)	0.0	0.0
Mortgage servicing rights impairment (recovery)	(3.4)	(6.9)	9.9
Deferred taxes	(4.7)	5.0	(6.6)
Net increase in cash surrender value of company-owned life insurance	(11.1)	(6.1)	(7.6)
Stock-based compensation expense	9.6	8.9	7.5
Originations of mortgage loans held for sale	(429.5)	(817.4)	(1,404.2)
Proceeds from sales of mortgage loans held for sale	461.6	883.9	1,468.4
Changes in operating assets and liabilities:
(Increase) decrease in interest receivable	(37.8)	3.7	(4.4)
Increase in other assets	(12.9)	(16.4)	(27.7)
Increase (decrease) in accrued interest payable	8.4	(2.0)	(6.3)
Increase (decrease) in accounts payable and accrued expenses	183.1	(1.7)	11.3
Net cash provided by operating activities	534.4	282.3	268.3
Purchases of investment securities:
Held-to-maturity	(858.5)	(1,238.0)	0.0
Available-for-sale	(3,309.4)	(2,717.8)	(2,444.1)
Proceeds from maturities, pay-downs, calls and sales of investment securities:
Held-to-maturity	370.5	257.6	40.4
Available-for-sale	1,926.5	1,118.0	1,441.4
Purchases of Federal Home Loan Bank and Federal Reserve Bank Stock	(241.4)	0.0	0.0
Proceeds from Federal Home Loan Bank and Federal Reserve Bank Stock	112.2	0.0	0.0
Proceeds from bank-owned life insurance settlements	1.3	1.0	5.0
Extensions of credit to clients, net of repayments	(951.4)	467.8	(894.6)
Proceeds from sales of OREO	3.4	1.7	10.1
Proceeds from the sale of Health Savings Accounts	1.4	0.0	0.0
Proceeds from sale of investments in unrelated entities	0.0	0.0	2.2
Acquisition of bank and bank holding company, net of cash and cash equivalents received	2,006.9	0.0	0.0
Capital expenditures, net of sales	(10.5)	(10.3)	(30.2)
Net cash used in investing activities	(949.0)	(2,120.0)	(1,869.8)
Cash flows from financing activities:
Net (decrease) increase in deposits	(2,884.0)	2,052.6	2,553.5
Net increase (decrease) in securities sold under repurchase agreements	28.9	(40.3)	393.8
Proceeds from (Repayments of) Other Debt	2,327.0	0.0	0.0
Repayments of Other Long-term Debt	164.1	0.0	0.1
Advances on long-term debt	14.3	0.0	98.6
Payments of Stock Issuance Costs	(0.8)	0.0	0.0
Proceeds from issuance of common stock	0.1	0.4	1.1
Purchase and retirement of common stock	(199.0)	(5.4)	(116.8)
Dividends paid to common stockholders	(182.1)	(101.6)	(128.6)
Net cash used in financing activities	(1,059.7)	1,905.7	2,801.5
Net (decrease) increase in cash and cash equivalents	(1,474.3)	68.0	1,200.0
Cash and cash equivalents at beginning of period	2,344.8	2,276.8	1,076.8
Cash and cash equivalents at end of period	870.5	2,344.8	2,276.8
Supplemental disclosures of cash flow information:
Cash paid during the period for income taxes	(57.8)	(56.8)	(54.4)
Cash paid during the period for interest expense	32.0	19.3	32.9
Supplemental disclosures of noncash investing and financing activities:
Transfer of held-to-maturity to available-for-sale securities	10.9	0.0	0.0
Non-cash transfer of securities from available-for-sale to held-to-maturity	463.6	672.2	0.0
Transfer of Loans Held-for-sale to Portfolio Loans	19.8	0.0	0.0
Amortization of unrealized gains and losses on transfers of securities	12.4	0.0	0.0
Right-of-use assets obtained in exchange for operating lease liabilities	22.9	5.9	3.6
Transfer of loans to other real estate owned	0.4	0.9	3.3
Asset Acquisition, Consideration Transferred, Equity Interest Issued and Issuable	1,723.3	0.0	0.0
Capitalization of internally originated mortgage servicing rights	2.5	3.7	11.7
Supplemental schedule of noncash investing activities from acquisitions:
Investment securities	2,699.0	0.0	0.0
Loans held for sale	217.0	0.0	0.0
Loans held for investment, net	7,645.5	0.0	0.0
Premises and equipment	144.7	0.0	0.0
Goodwill	479.3	0.0	0.0
Other intangibles	72.9	0.0	0.0
Mortgage servicing rights	1.3	0.0	0.0
Company-owned life insurance	186.6	0.0	0.0
Deferred tax assets	60.2	0.0	0.0
Other real estate owned	15.8	0.0	0.0
Other assets	200.8	0.0	0.0
Total noncash assets acquired	11,824.2	0.0	0.0
Deposits	11,688.0	0.0	0.0
Securities sold under repurchase agreements	74.0	0.0	0.0
Accounts payable and accrued expenses	110.4	0.0	0.0
Long-term debt	159.3	0.0	0.0
Subordinated debentures held by subsidiary trusts	76.1	0.0	0.0
Total liabilities assumed	(12,107.8)	0.0	0.0
Securities Purchased under Agreements to Resell, Increase	$ 101.1	0.0	0.0
Finite-Lived Intangible Assets, Gross		$ 106.0	$ 106.0