Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Estimated Fair Value
Financial assets:
Cash and cash equivalents	$ 698.6	$ 578.0
Investment debt securities held-to-maturity	2,480.5	2,874.0
Accrued interest receivable	127.3	129.1
Mortgage servicing rights, net	37.2	38.8
Loans held for sale	20.9	47.4
Net loans held for investment	17,257.5	17,334.4
Total financial assets	20,622.0	21,001.7
Financial liabilities:
Time deposits	2,864.3	2,981.7
Securities sold under repurchase agreements	557.2	782.7
Other Borrowings	2,080.0	2,603.0
Accrued interest payable	87.7	52.2
Long-term debt	132.1	115.6
Subordinated debentures held by subsidiary trusts	151.0	151.1
Total financial liabilities	25,858.2	26,999.5
Deposits, Excluding Time Deposits, Fair Value Disclosure	19,985.9	20,313.2
Carrying Amount
Financial assets:
Cash and cash equivalents	698.6	578.0
Investment debt securities held-to-maturity	2,741.3	3,207.9
Accrued interest receivable	127.3	129.1
Mortgage servicing rights, net	26.3	28.3
Loans held for sale	20.9	47.4
Net loans held for investment	17,801.7	18,051.9
Total financial assets	21,416.1	22,042.6
Financial liabilities:
Time deposits	2,878.2	3,009.9
Securities sold under repurchase agreements	557.2	782.7
Other Borrowings	2,080.0	2,603.0
Accrued interest payable	87.7	52.2
Long-term debt	137.3	120.8
Subordinated debentures held by subsidiary trusts	163.1	163.1
Total financial liabilities	25,889.4	27,044.9
Deposits, Excluding Time Deposits, Fair Value Disclosure	19,985.9	20,313.2
Investment debt securities held-to-maturity	2,480.5	2,874.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	698.6	578.0
Investment debt securities held-to-maturity	0.0	0.0
Accrued interest receivable	0.0	0.0
Mortgage servicing rights, net	0.0	0.0
Loans held for sale	0.0	0.0
Net loans held for investment	0.0	0.0
Total financial assets	698.6	578.0
Financial liabilities:
Time deposits	0.0	0.0
Securities sold under repurchase agreements	0.0	0.0
Other Borrowings	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	19,985.9	20,313.2
Deposits, Excluding Time Deposits, Fair Value Disclosure	19,985.9	20,313.2
Significant Other Observable Inputs (Level 2) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0.0	0.0
Investment debt securities held-to-maturity	2,480.5	2,874.0
Accrued interest receivable	127.3	129.1
Mortgage servicing rights, net	37.2	38.8
Loans held for sale	2.1	0.5
Net loans held for investment	17,147.9	17,281.8
Total financial assets	19,795.0	20,324.2
Financial liabilities:
Time deposits	2,864.3	2,981.7
Securities sold under repurchase agreements	557.2	782.7
Other Borrowings	2,080.0	2,603.0
Accrued interest payable	87.7	52.2
Long-term debt	132.1	115.6
Subordinated debentures held by subsidiary trusts	151.0	151.1
Total financial liabilities	5,872.3	6,686.3
Deposits, Excluding Time Deposits, Fair Value Disclosure	0.0	0.0
Significant Unobservable Inputs (Level 3) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0.0	0.0
Investment debt securities held-to-maturity	0.0	0.0
Accrued interest receivable	0.0	0.0
Mortgage servicing rights, net	0.0	0.0
Loans held for sale	18.8	46.9
Net loans held for investment	109.6	52.6
Total financial assets	128.4	99.5
Financial liabilities:
Time deposits	0.0	0.0
Securities sold under repurchase agreements	0.0	0.0
Other Borrowings	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
Deposits, Excluding Time Deposits, Fair Value Disclosure	$ 0.0	$ 0.0