Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Financial assets:
Investment debt securities held-to-maturity	$ 2,924.7	$ 3,052.2
Derivative assets	40.2	45.2
Financial liabilities:
Derivative liabilities	163.1	159.6
Estimated Fair Value
Financial assets:
Cash and cash equivalents	680.5	870.5
Investment debt securities available-for-sale	5,857.9	6,946.1
Investment debt securities held-to-maturity	2,924.7	3,052.2
Accrued interest receivable	119.1	118.3
Mortgage servicing rights, net	38.2	37.4
Loans held for sale	76.5	79.9
Net loans held for investment	17,519.5	17,552.1
Derivative assets	40.2	45.2
Deferred compensation plan assets	18.4	18.7
Total financial assets	27,275.0	28,720.4
Financial liabilities:
Total deposits, excluding time deposits	20,836.8	23,145.2
Time deposits	2,695.4	1,876.1
Securities sold under repurchase agreements	929.9	1,052.9
Other Borrowings	2,589.0	2,327.0
Accrued interest payable	45.8	14.5
Long-term debt	112.3	116.3
Subordinated debentures held by subsidiary trusts	154.0	155.8
Derivative liabilities	163.1	159.6
Deferred compensation plan liabilities	18.4	18.7
Total financial liabilities	27,544.7	28,866.1
Carrying Amount
Financial assets:
Cash and cash equivalents	680.5	870.5
Investment debt securities available-for-sale	5,857.9	6,946.1
Investment debt securities held-to-maturity	3,317.7	3,451.8
Accrued interest receivable	119.1	118.3
Mortgage servicing rights, net	29.8	31.1
Loans held for sale	76.5	79.9
Net loans held for investment	18,038.8	17,879.1
Derivative assets	40.2	45.2
Deferred compensation plan assets	18.4	18.7
Total financial assets	28,178.9	29,440.7
Financial liabilities:
Total deposits, excluding time deposits	20,836.8	23,145.2
Time deposits	2,742.4	1,928.4
Securities sold under repurchase agreements	929.9	1,052.9
Other Borrowings	2,589.0	2,327.0
Accrued interest payable	45.8	14.5
Long-term debt	120.8	120.8
Subordinated debentures held by subsidiary trusts	163.1	163.1
Derivative liabilities	163.1	159.6
Deferred compensation plan liabilities	18.4	18.7
Total financial liabilities	27,609.3	28,930.2
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	680.5	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	680.5	870.5
Financial liabilities:
Total deposits, excluding time deposits	20,836.8	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	20,836.8	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	5,857.9	6,946.1
Investment debt securities held-to-maturity	2,924.7	3,052.2
Accrued interest receivable	119.1	118.3
Mortgage servicing rights, net	38.2	37.4
Loans held for sale	11.0	6.9
Net loans held for investment	17,456.0	17,513.0
Derivative assets	40.2	45.2
Deferred compensation plan assets	18.4	18.7
Total financial assets	26,465.5	27,737.8
Financial liabilities:
Total deposits, excluding time deposits	0.0	0.0
Time deposits	2,695.4	1,876.1
Securities sold under repurchase agreements	929.9	1,052.9
Other Borrowings	2,589.0	2,327.0
Accrued interest payable	45.8	14.5
Long-term debt	112.3	116.3
Subordinated debentures held by subsidiary trusts	154.0	155.8
Derivative liabilities	163.1	159.6
Deferred compensation plan liabilities	18.4	18.7
Total financial liabilities	6,707.9	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	65.5	73.0
Net loans held for investment	63.5	39.1
Derivative assets	0.0	0.0
Deferred compensation plan assets	0.0	0.0
Total financial assets	129.0	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