Consolidated Balance Sheets - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Assets [Abstract]
Cash and due from banks	$ 321.7	$ 358.2
Interest bearing deposits in banks	886.9	951.4
Federal funds sold	0.1	0.1
Total cash and cash equivalents	1,208.7	1,309.7
Investment securities:
Available-for-sale, at fair value	5,716.6	5,288.1
Held-to-maturity (estimated fair values of $2,082.7 at March 31, 2026 and $2,136.6 at December 31, 2025)	2,293.4	2,342.1
Total investment securities	8,010.0	7,630.2
Federal Home Loan Bank Stock and Federal Reserve Bank Stock	106.3	106.3
Loans held for sale ($4.7 and $1.1 of which is recorded at fair value at March 31, 2026 and December 31, 2025, respectively)	70.8	73.6
Allowance for credit losses	(195.8)	(191.4)
Net loans held for investment	14,532.6	15,010.2
Goodwill	1,100.9	1,100.9
Bank Owned Life Insurance	524.6	523.0
Premises and equipment, net of accumulated depreciation	403.1	406.6
Other intangibles, net of accumulated amortization	50.0	53.3
Accrued interest receivable	101.3	102.6
Mortgage servicing rights, net of accumulated amortization	22.5	23.1
Real Estate Acquired Through Foreclosure	6.6	3.4
Deferred tax assets	57.9	59.6
Other assets	231.5	238.1
Total assets	26,426.8	26,640.6
Deposits:
Noninterest bearing	5,229.0	5,286.8
Interest bearing	16,654.0	16,801.5
Total deposits	21,883.0	22,088.3
Securities sold under repurchase agreements	476.1	479.6
Accounts payable and accrued expenses	373.7	286.8
Accrued interest payable	33.1	36.9
Long-term debt	146.7	146.3
Allowance for credit losses on off-balance sheet credit exposures	5.8	5.9
Subordinated debentures held by subsidiary trusts	149.9	149.8
Total liabilities	$ 23,068.3	$ 23,193.6
Preferred Stock, Shares Authorized	100,000	100,000
Preferred Stock, Par or Stated Value Per Share	$ 0.00001	$ 0.00001
Common Stock, Shares, Issued	97,446,230	101,105,745
Stockholders’ equity:
Preferred stock, $0.00001 par value; 100,000 shares authorized at March 31, 2026 and December 31, 2025; zero issued and outstanding, respectively	$ 0.0	$ 0.0
Common stock and additional paid-in-capital, $0.00001 par value; 150,000,000 shares authorized at March 31, 2026 and December 31, 2025; 97,446,230 and 101,105,745 shares issued and outstanding, respectively	2,265.5	2,350.9
Retained earnings	1,288.7	1,274.2
Accumulated other comprehensive loss, net	(195.7)	(178.1)
Total stockholders’ equity	3,358.5	3,447.0
Total liabilities and stockholders’ equity	$ 26,426.8	$ 26,640.6
Common Stock, Shares Authorized	150,000,000	150,000,000
Shares outstanding (in shares)	97,446,230	101,105,745