Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
Financial assets:
Held-to-maturity investment securities, estimated fair value	$ 2,136.6	$ 2,358.6
Derivative Assets, Estimated Fair Value	26.6	44.9
Deferred Compensation Plan Assets	23.2	22.7
Financial liabilities:
Derivative Liabilities, Estimated Fair Value	77.3	133.8
Deferred Compensation Liability, Current and Noncurrent	23.2	22.7
Carrying Amount
Financial assets:
Cash and cash equivalents	1,309.7	896.6
Held-to-maturity investment securities, estimated fair value	2,342.1	2,687.5
Accrued interest receivable	102.6	116.8
Mortgage servicing rights, net	23.1	25.7
Net loans held for investment	15,010.2	17,640.8
Total financial assets	18,787.7	21,367.4
Financial liabilities:
Total deposits, excluding time deposits	19,450.0	20,125.1
Time deposits	2,638.3	2,890.5
Securities sold under repurchase agreements	479.6	523.9
Federal Home Loan Bank Borrowings, Fair Value Disclosure	0.0	1,567.5
Accrued interest payable	36.9	47.0
Long-term debt	146.3	132.2
Subordinated debentures held by subsidiary trusts	149.8	163.1
Total financial liabilities	22,900.9	25,449.3
Estimated Fair Value
Financial assets:
Cash and cash equivalents	1,309.7	896.6
Held-to-maturity investment securities, estimated fair value	2,136.6	2,358.6
Accrued interest receivable	102.6	116.8
Mortgage servicing rights, net	32.3	35.4
Net loans held for investment	14,769.1	17,154.7
Total financial assets	18,350.3	20,562.1
Financial liabilities:
Total deposits, excluding time deposits	19,450.0	20,125.1
Time deposits	2,629.2	2,875.5
Securities sold under repurchase agreements	479.6	523.9
Federal Home Loan Bank Borrowings, Fair Value Disclosure	0.0	1,567.5
Accrued interest payable	36.9	47.0
Long-term debt	153.4	131.9
Subordinated debentures held by subsidiary trusts	140.0	152.3
Total financial liabilities	22,889.1	25,423.2
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	1,309.7	896.6
Held-to-maturity investment securities, estimated fair value	0.0	0.0
Accrued interest receivable	0.0	0.0
Mortgage servicing rights, net	0.0	0.0
Net loans held for investment	0.0	0.0
Total financial assets	1,309.7	896.6
Financial liabilities:
Total deposits, excluding time deposits	19,450.0	20,125.1
Time deposits	0.0	0.0
Securities sold under repurchase agreements	0.0	0.0
Federal Home Loan Bank Borrowings, Fair Value Disclosure	0.0	0.0
Accrued interest payable	0.0	0.0
Long-term debt	0.0	0.0
Subordinated debentures held by subsidiary trusts	0.0	0.0
Total financial liabilities	19,450.0	20,125.1
Significant Other Observable Inputs (Level 2) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0.0	0.0
Held-to-maturity investment securities, estimated fair value	2,136.6	2,358.6
Accrued interest receivable	102.6	116.8
Mortgage servicing rights, net	0.0	0.0
Net loans held for investment	0.0	0.0
Total financial assets	2,239.2	2,475.4
Financial liabilities:
Total deposits, excluding time deposits	0.0	0.0
Time deposits	2,629.2	2,875.5
Securities sold under repurchase agreements	479.6	523.9
Federal Home Loan Bank Borrowings, Fair Value Disclosure	0.0	1,567.5
Accrued interest payable	36.9	47.0
Long-term debt	153.4	131.9
Subordinated debentures held by subsidiary trusts	140.0	152.3
Total financial liabilities	3,439.1	5,298.1
Significant Unobservable Inputs (Level 3) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0.0	0.0
Held-to-maturity investment securities, estimated fair value	0.0	0.0
Accrued interest receivable	0.0	0.0
Mortgage servicing rights, net	32.3	35.4
Net loans held for investment	14,769.1	17,154.7
Total financial assets	14,801.4	17,190.1
Financial liabilities:
Total deposits, excluding time deposits	0.0	0.0
Time deposits	0.0	0.0
Securities sold under repurchase agreements	0.0	0.0
Federal Home Loan Bank Borrowings, Fair Value Disclosure	0.0	0.0
Accrued interest payable	0.0	0.0
Long-term debt	0.0	0.0
Subordinated debentures held by subsidiary trusts	0.0	0.0
Total financial liabilities	$ 0.0	$ 0.0