Consolidated Balance Sheets - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Assets [Abstract]
Cash and due from banks	$ 390.4	$ 378.0
Interest bearing deposits in banks	480.9	518.5
Federal funds sold	0.1	0.1
Total cash and cash equivalents	871.4	896.6
Investment securities:
Available-for-sale, at fair value	4,864.3	5,057.1
Held-to-maturity (estimated fair values of $2,361.2 at March 31, 2025 and $2,358.6 at December 31, 2024)	2,639.5	2,687.5
Total investment securities	7,503.8	7,744.6
Federal Home Loan Bank Stock and Federal Reserve Bank Stock	150.1	177.4
Loans held for sale, at fair value	0.4	0.9
Allowance for credit losses	(215.3)	(204.1)
Net loans held for investment	17,162.0	17,640.8
Goodwill	1,100.9	1,100.9
Bank Owned Life Insurance	514.2	513.0
Premises and equipment, net of accumulated depreciation	428.9	427.2
Other intangibles, net of accumulated amortization	63.5	66.8
Accrued interest receivable	107.9	116.8
Mortgage servicing rights, net of accumulated amortization	24.9	25.7
Real Estate Acquired Through Foreclosure	3.5	4.3
Deferred tax assets	92.5	118.4
Other assets	255.8	304.0
Total assets	28,279.8	29,137.4
Deposits:
Noninterest bearing	5,590.2	5,797.6
Interest bearing	17,142.6	17,218.0
Total deposits	22,732.8	23,015.6
Securities sold under repurchase agreements	528.0	523.9
Accounts payable and accrued expenses	358.5	378.9
Accrued interest payable	40.8	47.0
Long-term debt	130.2	132.2
Other Short-term Borrowings	960.0	1,567.5
Allowance for credit losses on off-balance sheet credit exposures	5.1	5.2
Subordinated debentures held by subsidiary trusts	163.1	163.1
Total liabilities	$ 24,918.5	$ 25,833.4
Preferred Stock, Shares Authorized	100,000	100,000
Preferred Stock, Par or Stated Value Per Share	$ 0.00001	$ 0.00001
Common Stock, Shares, Issued	104,909,761	104,585,964
Stockholders’ equity:
Preferred stock, $0.00001 par value; 100,000 shares authorized at March 31, 2025 and December 31, 2024; 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, 2025 and December 31, 2024; 104,909,761 and 104,585,964 shares issued and outstanding, respectively	2,460.2	2,459.5
Retained earnings	1,168.6	1,166.4
Accumulated other comprehensive loss, net	(267.5)	(321.9)
Total stockholders’ equity	3,361.3	3,304.0
Total liabilities and stockholders’ equity	$ 28,279.8	$ 29,137.4
Common Stock, Shares Authorized	150,000,000	150,000,000
Shares outstanding (in shares)	104,909,761	104,585,964