Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Financial assets:
Investment debt securities held-to-maturity	$ 2,796.1	$ 3,052.2
Derivative assets	57.4	45.2
Financial liabilities:
Derivative liabilities	210.0	159.6
Estimated Fair Value
Financial assets:
Cash and cash equivalents	593.1	870.5
Investment debt securities available-for-sale	5,627.7	6,946.1
Investment debt securities held-to-maturity	2,796.1	3,052.2
Accrued interest receivable	129.2	118.3
Mortgage servicing rights, net	35.6	37.4
Loans held for sale	59.1	79.9
Net loans held for investment	17,226.1	17,552.1
Derivative assets	57.4	45.2
Deferred compensation plan assets	19.2	18.7
Total financial assets	26,543.5	28,720.4
Financial liabilities:
Total deposits, excluding time deposits	20,516.8	23,145.2
Time deposits	3,126.2	1,876.1
Securities sold under repurchase agreements	889.5	1,052.9
Other Borrowings	2,067.0	2,327.0
Accrued interest payable	63.7	14.5
Long-term debt	114.4	116.3
Subordinated debentures held by subsidiary trusts	152.5	155.8
Derivative liabilities	210.0	159.6
Deferred compensation plan liabilities	19.2	18.7
Total financial liabilities	27,159.3	28,866.1
Carrying Amount
Financial assets:
Cash and cash equivalents	593.1	870.5
Investment debt securities available-for-sale	5,627.7	6,946.1
Investment debt securities held-to-maturity	3,259.5	3,451.8
Accrued interest receivable	129.2	118.3
Mortgage servicing rights, net	29.1	31.1
Loans held for sale	59.1	79.9
Net loans held for investment	17,986.6	17,879.1
Derivative assets	57.4	45.2
Deferred compensation plan assets	19.2	18.7
Total financial assets	27,760.9	29,440.7
Financial liabilities:
Total deposits, excluding time deposits	20,516.8	23,145.2
Time deposits	3,162.7	1,928.4
Securities sold under repurchase agreements	889.5	1,052.9
Other Borrowings	2,067.0	2,327.0
Accrued interest payable	63.7	14.5
Long-term debt	120.8	120.8
Subordinated debentures held by subsidiary trusts	163.1	163.1
Derivative liabilities	210.0	159.6
Deferred compensation plan liabilities	19.2	18.7
Total financial liabilities	27,212.8	28,930.2
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	593.1	870.5
Investment debt securities available-for-sale	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	0.0	0.0
Net loans held for investment	0.0	0.0
Derivative assets	0.0	0.0
Deferred compensation plan assets	0.0	0.0
Total financial assets	593.1	870.5
Financial liabilities:
Total deposits, excluding time deposits	20,516.8	23,145.2
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
Derivative liabilities	0.0	0.0
Deferred compensation plan liabilities	0.0	0.0
Total financial liabilities	20,516.8	23,145.2
Significant Other Observable Inputs (Level 2) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0.0	0.0
Investment debt securities available-for-sale	5,627.7	6,946.1
Investment debt securities held-to-maturity	2,796.1	3,052.2
Accrued interest receivable	129.2	118.3
Mortgage servicing rights, net	35.6	37.4
Loans held for sale	3.2	6.9
Net loans held for investment	17,169.8	17,513.0
Derivative assets	57.4	45.2
Deferred compensation plan assets	19.2	18.7
Total financial assets	25,838.2	27,737.8
Financial liabilities:
Total deposits, excluding time deposits	0.0	0.0
Time deposits	3,126.2	1,876.1
Securities sold under repurchase agreements	889.5	1,052.9
Other Borrowings	2,067.0	2,327.0
Accrued interest payable	63.7	14.5
Long-term debt	114.4	116.3
Subordinated debentures held by subsidiary trusts	152.5	155.8
Derivative liabilities	210.0	159.6
Deferred compensation plan liabilities	19.2	18.7
Total financial liabilities	6,642.5	5,720.9
Significant Unobservable Inputs (Level 3) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0.0	0.0
Investment debt securities available-for-sale	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	55.9	73.0
Net loans held for investment	56.3	39.1
Derivative assets	0.0	0.0
Deferred compensation plan assets	0.0	0.0
Total financial assets	112.2	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
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
Derivative liabilities	0.0	0.0
Deferred compensation plan liabilities	0.0	0.0
Total financial liabilities	$ 0.0	$ 0.0