Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Financial assets:
Investment debt securities held-to-maturity	$ 3,025.8	$ 3,052.2
Derivative assets	43.1	45.2
Financial liabilities:
Derivative liabilities	126.7	159.6
Estimated Fair Value
Financial assets:
Cash and cash equivalents	1,080.7	870.5
Investment debt securities available-for-sale	6,049.3	6,946.1
Investment debt securities held-to-maturity	3,025.8	3,052.2
Accrued interest receivable	113.7	118.3
Mortgage servicing rights, net	38.1	37.4
Loans held for sale	80.9	79.9
Net loans held for investment	17,585.5	17,552.1
Derivative assets	43.1	45.2
Deferred compensation plan assets	17.3	18.7
Total financial assets	28,034.4	28,720.4
Financial liabilities:
Total deposits, excluding time deposits	21,858.1	23,145.2
Time deposits	2,198.8	1,876.1
Securities sold under repurchase agreements	970.8	1,052.9
Other Borrowings	2,710.0	2,327.0
Accrued interest payable	26.1	14.5
Long-term debt	114.5	116.3
Subordinated debentures held by subsidiary trusts	157.8	155.8
Derivative liabilities	126.7	159.6
Deferred compensation plan liabilities	17.3	18.7
Total financial liabilities	28,180.1	28,866.1
Carrying Amount
Financial assets:
Cash and cash equivalents	1,080.7	870.5
Investment debt securities available-for-sale	6,049.3	6,946.1
Investment debt securities held-to-maturity	3,376.2	3,451.8
Accrued interest receivable	113.7	118.3
Mortgage servicing rights, net	30.1	31.1
Loans held for sale	80.9	79.9
Net loans held for investment	18,019.6	17,879.1
Derivative assets	43.1	45.2
Deferred compensation plan assets	17.3	18.7
Total financial assets	28,810.9	29,440.7
Financial liabilities:
Total deposits, excluding time deposits	21,858.1	23,145.2
Time deposits	2,248.9	1,928.4
Securities sold under repurchase agreements	970.8	1,052.9
Other Borrowings	2,710.0	2,327.0
Accrued interest payable	26.1	14.5
Long-term debt	120.8	120.8
Subordinated debentures held by subsidiary trusts	163.1	163.1
Derivative liabilities	126.7	159.6
Deferred compensation plan liabilities	17.3	18.7
Total financial liabilities	28,241.8	28,930.2
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	1,080.7	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	1,080.7	870.5
Financial liabilities:
Total deposits, excluding time deposits	21,858.1	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	21,858.1	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	6,049.3	6,946.1
Investment debt securities held-to-maturity	3,025.8	3,052.2
Accrued interest receivable	113.7	118.3
Mortgage servicing rights, net	38.1	37.4
Loans held for sale	8.5	6.9
Net loans held for investment	17,526.0	17,513.0
Derivative assets	43.1	45.2
Deferred compensation plan assets	17.3	18.7
Total financial assets	26,821.8	27,737.8
Financial liabilities:
Total deposits, excluding time deposits	0.0	0.0
Time deposits	2,198.8	1,876.1
Securities sold under repurchase agreements	970.8	1,052.9
Other Borrowings	2,710.0	2,327.0
Accrued interest payable	26.1	14.5
Long-term debt	114.5	116.3
Subordinated debentures held by subsidiary trusts	157.8	155.8
Derivative liabilities	126.7	159.6
Deferred compensation plan liabilities	17.3	18.7
Total financial liabilities	6,322.0	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	72.4	73.0
Net loans held for investment	59.5	39.1
Derivative assets	0.0	0.0
Deferred compensation plan assets	0.0	0.0
Total financial assets	131.9	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