Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Financial assets:
Held-to-maturity investment securities, estimated fair value	$ 2,874.0	$ 3,052.2
Derivative Assets, Estimated Fair Value	37.4	45.2
Deferred compensation plan assets	19.2	18.7
Financial liabilities:
Derivative Liabilities, Estimated Fair Value	127.3	159.6
Deferred compensation plan liabilities	19.2	18.7
Carrying Amount
Financial assets:
Cash and cash equivalents	578.0	870.5
Estimated Fair Value	5,841.5	6,946.1
Held-to-maturity investment securities, estimated fair value	3,207.9	3,451.8
Accrued interest receivable	129.1	118.3
Mortgage servicing rights, net	28.3	31.1
Loans Held-for-sale, Fair Value Disclosure	47.4	79.9
Net loans held for investment	18,051.9	17,879.1
Derivative Assets, Estimated Fair Value	37.4	45.2
Deferred compensation plan assets	19.2	18.7
Total financial assets	27,940.7	29,440.7
Financial liabilities:
Total deposits, excluding time deposits	20,313.2	23,145.2
Time deposits	3,009.9	1,928.4
Securities sold under repurchase agreements	782.7	1,052.9
Federal Home Loan Bank Borrowings, Fair Value Disclosure	2,603.0	2,327.0
Accrued interest payable	52.2	14.5
Long-term debt	120.8	120.8
Subordinated debentures held by subsidiary trusts	163.1	163.1
Derivative Liabilities, Estimated Fair Value	127.3	159.6
Deferred compensation plan liabilities	19.2	18.7
Total financial liabilities	27,191.4	28,930.2
Estimated Fair Value
Financial assets:
Cash and cash equivalents	578.0	870.5
Estimated Fair Value	5,841.5	6,946.1
Held-to-maturity investment securities, estimated fair value	2,874.0	3,052.2
Accrued interest receivable	129.1	118.3
Mortgage servicing rights, net	38.8	37.4
Loans Held-for-sale, Fair Value Disclosure	47.4	79.9
Net loans held for investment	17,334.4	17,552.1
Derivative Assets, Estimated Fair Value	37.4	45.2
Deferred compensation plan assets	19.2	18.7
Total financial assets	26,899.8	28,720.4
Financial liabilities:
Total deposits, excluding time deposits	20,313.2	23,145.2
Time deposits	2,981.7	1,876.1
Securities sold under repurchase agreements	782.7	1,052.9
Federal Home Loan Bank Borrowings, Fair Value Disclosure	2,603.0	2,327.0
Accrued interest payable	52.2	14.5
Long-term debt	115.6	116.3
Subordinated debentures held by subsidiary trusts	151.1	155.8
Derivative Liabilities, Estimated Fair Value	127.3	159.6
Deferred compensation plan liabilities	19.2	18.7
Total financial liabilities	27,146.0	28,866.1
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	578.0	870.5
Estimated Fair Value	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
Loans Held-for-sale, Fair Value Disclosure	0.0	0.0
Net loans held for investment	0.0	0.0
Derivative Assets, Estimated Fair Value	0.0	0.0
Deferred compensation plan assets	0.0	0.0
Total financial assets	578.0	870.5
Financial liabilities:
Total deposits, excluding time deposits	20,313.2	23,145.2
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
Derivative Liabilities, Estimated Fair Value	0.0	0.0
Deferred compensation plan liabilities	0.0	0.0
Total financial liabilities	20,313.2	23,145.2
Significant Other Observable Inputs (Level 2) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0.0	0.0
Estimated Fair Value	5,841.5	6,946.1
Held-to-maturity investment securities, estimated fair value	2,874.0	3,052.2
Accrued interest receivable	129.1	118.3
Mortgage servicing rights, net	38.8	37.4
Loans Held-for-sale, Fair Value Disclosure	0.5	6.9
Net loans held for investment	17,281.8	17,513.0
Derivative Assets, Estimated Fair Value	37.4	45.2
Deferred compensation plan assets	19.2	18.7
Total financial assets	26,222.3	27,737.8
Financial liabilities:
Total deposits, excluding time deposits	0.0	0.0
Time deposits	2,981.7	1,876.1
Securities sold under repurchase agreements	782.7	1,052.9
Federal Home Loan Bank Borrowings, Fair Value Disclosure	2,603.0	2,327.0
Accrued interest payable	52.2	14.5
Long-term debt	115.6	116.3
Subordinated debentures held by subsidiary trusts	151.1	155.8
Derivative Liabilities, Estimated Fair Value	127.3	159.6
Deferred compensation plan liabilities	19.2	18.7
Total financial liabilities	6,832.8	5,720.9
Significant Unobservable Inputs (Level 3) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0.0	0.0
Estimated Fair Value	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
Loans Held-for-sale, Fair Value Disclosure	46.9	73.0
Net loans held for investment	52.6	39.1
Derivative Assets, Estimated Fair Value	0.0	0.0
Deferred compensation plan assets	0.0	0.0
Total financial assets	99.5	112.1
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
Derivative Liabilities, Estimated Fair Value	0.0	0.0
Deferred compensation plan liabilities	0.0	0.0
Total financial liabilities	$ 0.0	$ 0.0