Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Financial assets:
Investment debt securities held-to-maturity	$ 2,440.4	$ 2,874.0
Derivative assets	45.0	37.4
Financial liabilities:
Derivative liabilities	157.2	127.3
Estimated Fair Value
Financial assets:
Cash and cash equivalents	958.5	578.0
Investment debt securities available-for-sale	5,603.1	5,841.5
Investment debt securities held-to-maturity	2,440.4	2,874.0
Accrued interest receivable	126.4	129.1
Mortgage servicing rights, net	38.9	38.8
Loans held for sale	22.3	47.4
Net loans held for investment	17,259.6	17,334.4
Derivative assets	45.0	37.4
Deferred compensation plan assets	20.6	19.2
Total financial assets	26,514.8	26,899.8
Financial liabilities:
Total deposits, excluding time deposits	20,029.9	20,313.2
Time deposits	2,814.2	2,981.7
Securities sold under repurchase agreements	741.8	782.7
Other Borrowings	2,430.0	2,603.0
Accrued interest payable	71.1	52.2
Long-term debt	380.7	115.6
Subordinated debentures held by subsidiary trusts	153.9	151.1
Derivative liabilities	157.2	127.3
Deferred compensation plan liabilities	20.6	19.2
Total financial liabilities	26,799.4	27,146.0
Carrying Amount
Financial assets:
Cash and cash equivalents	958.5	578.0
Investment debt securities available-for-sale	5,603.1	5,841.5
Investment debt securities held-to-maturity	2,798.5	3,207.9
Accrued interest receivable	126.4	129.1
Mortgage servicing rights, net	27.0	28.3
Loans held for sale	22.3	47.4
Net loans held for investment	18,002.2	18,051.9
Derivative assets	45.0	37.4
Deferred compensation plan assets	20.6	19.2
Total financial assets	27,603.6	27,940.7
Financial liabilities:
Total deposits, excluding time deposits	20,029.9	20,313.2
Time deposits	2,840.8	3,009.9
Securities sold under repurchase agreements	741.8	782.7
Other Borrowings	2,430.0	2,603.0
Accrued interest payable	71.1	52.2
Long-term debt	383.4	120.8
Subordinated debentures held by subsidiary trusts	163.1	163.1
Derivative liabilities	157.2	127.3
Deferred compensation plan liabilities	20.6	19.2
Total financial liabilities	26,837.9	27,191.4
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	958.5	578.0
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	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	958.5	578.0
Financial liabilities:
Total deposits, excluding time deposits	20,029.9	20,313.2
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	20,029.9	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 available-for-sale	5,603.1	5,841.5
Investment debt securities held-to-maturity	2,440.4	2,874.0
Accrued interest receivable	126.4	129.1
Mortgage servicing rights, net	38.9	38.8
Loans held for sale	3.1	0.5
Net loans held for investment	17,155.7	17,281.8
Derivative assets	45.0	37.4
Deferred compensation plan assets	20.6	19.2
Total financial assets	25,433.2	26,222.3
Financial liabilities:
Total deposits, excluding time deposits	0.0	0.0
Time deposits	2,814.2	2,981.7
Securities sold under repurchase agreements	741.8	782.7
Other Borrowings	2,430.0	2,603.0
Accrued interest payable	71.1	52.2
Long-term debt	380.7	115.6
Subordinated debentures held by subsidiary trusts	153.9	151.1
Derivative liabilities	157.2	127.3
Deferred compensation plan liabilities	20.6	19.2
Total financial liabilities	6,769.5	6,832.8
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	19.2	46.9
Net loans held for investment	103.9	52.6
Derivative assets	0.0	0.0
Deferred compensation plan assets	0.0	0.0
Total financial assets	123.1	99.5
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