Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Financial assets:
Held-to-maturity investment securities, estimated fair value	$ 2,358.6	$ 2,874.0
Derivative Assets, Estimated Fair Value	44.9	37.4
Deferred Compensation Plan Assets	22.7	19.2
Financial liabilities:
Derivative Liabilities, Estimated Fair Value	133.8	127.3
Deferred Compensation Liability, Current and Noncurrent	22.7	19.2
Carrying Amount
Financial assets:
Cash and cash equivalents	896.6	578.0
Held-to-maturity investment securities, estimated fair value	2,687.5	3,207.9
Accrued interest receivable	116.8	129.1
Mortgage servicing rights, net	25.7	28.3
Net loans held for investment	17,640.8	18,051.9
Total financial assets	21,367.4	21,995.2
Financial liabilities:
Total deposits, excluding time deposits	20,125.1	20,313.2
Time deposits	2,890.5	3,009.9
Securities sold under repurchase agreements	523.9	782.7
Federal Home Loan Bank Borrowings, Fair Value Disclosure	1,567.5	2,603.0
Accrued interest payable	47.0	52.2
Long-term debt	132.2	120.8
Subordinated debentures held by subsidiary trusts	163.1	163.1
Total financial liabilities	25,449.3	27,044.9
Estimated Fair Value
Financial assets:
Cash and cash equivalents	896.6	578.0
Held-to-maturity investment securities, estimated fair value	2,358.6	2,874.0
Accrued interest receivable	116.8	129.1
Mortgage servicing rights, net	35.4	38.8
Net loans held for investment	17,154.7	17,334.4
Total financial assets	20,562.1	20,954.3
Financial liabilities:
Total deposits, excluding time deposits	20,125.1	20,313.2
Time deposits	2,875.5	2,981.7
Securities sold under repurchase agreements	523.9	782.7
Federal Home Loan Bank Borrowings, Fair Value Disclosure	1,567.5	2,603.0
Accrued interest payable	47.0	52.2
Long-term debt	131.9	115.6
Subordinated debentures held by subsidiary trusts	152.3	151.1
Total financial liabilities	25,423.2	26,999.5
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	896.6	578.0
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	896.6	578.0
Financial liabilities:
Total deposits, excluding time deposits	20,125.1	20,313.2
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	20,125.1	20,313.2
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	2,358.6	2,874.0
Accrued interest receivable	116.8	129.1
Mortgage servicing rights, net	35.4	38.8
Net loans held for investment	0.0	17,281.8
Total financial assets	2,510.8	20,323.7
Financial liabilities:
Total deposits, excluding time deposits	0.0	0.0
Time deposits	2,875.5	2,981.7
Securities sold under repurchase agreements	523.9	782.7
Federal Home Loan Bank Borrowings, Fair Value Disclosure	1,567.5	2,603.0
Accrued interest payable	47.0	52.2
Long-term debt	131.9	115.6
Subordinated debentures held by subsidiary trusts	152.3	151.1
Total financial liabilities	5,298.1	6,686.3
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	0.0	0.0
Net loans held for investment	17,154.7	52.6
Total financial assets	17,154.7	52.6
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