Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Financial assets:
Investment debt securities held-to-maturity	$ 2,497.8	$ 2,874.0
Derivative assets	44.2	37.4
Financial liabilities:
Derivative liabilities	156.6	127.3
Estimated Fair Value
Financial assets:
Cash and cash equivalents	635.0	578.0
Investment debt securities available-for-sale	5,773.3	5,841.5
Investment debt securities held-to-maturity	2,497.8	2,874.0
Accrued interest receivable	125.5	129.1
Mortgage servicing rights, net	40.0	38.8
Loans held for sale	22.7	47.4
Net loans held for investment	17,357.4	17,334.4
Derivative assets	44.2	37.4
Deferred compensation plan assets	19.9	19.2
Total financial assets	26,515.8	26,899.8
Financial liabilities:
Total deposits, excluding time deposits	19,876.1	20,313.2
Time deposits	2,906.3	2,981.7
Securities sold under repurchase agreements	794.2	782.7
Other Borrowings	2,342.0	2,603.0
Accrued interest payable	53.6	52.2
Long-term debt	367.9	115.6
Subordinated debentures held by subsidiary trusts	154.6	151.1
Derivative liabilities	156.6	127.3
Deferred compensation plan liabilities	19.9	19.2
Total financial liabilities	26,671.2	27,146.0
Carrying Amount
Financial assets:
Cash and cash equivalents	635.0	578.0
Investment debt securities available-for-sale	5,773.3	5,841.5
Investment debt securities held-to-maturity	2,852.8	3,207.9
Accrued interest receivable	125.5	129.1
Mortgage servicing rights, net	27.6	28.3
Loans held for sale	22.7	47.4
Net loans held for investment	17,975.1	18,051.9
Derivative assets	44.2	37.4
Deferred compensation plan assets	19.9	19.2
Total financial assets	27,476.1	27,940.7
Financial liabilities:
Total deposits, excluding time deposits	19,876.1	20,313.2
Time deposits	2,933.9	3,009.9
Securities sold under repurchase agreements	794.2	782.7
Other Borrowings	2,342.0	2,603.0
Accrued interest payable	53.6	52.2
Long-term debt	370.8	120.8
Subordinated debentures held by subsidiary trusts	163.1	163.1
Derivative liabilities	156.6	127.3
Deferred compensation plan liabilities	19.9	19.2
Total financial liabilities	26,710.2	27,191.4
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	635.0	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	635.0	578.0
Financial liabilities:
Total deposits, excluding time deposits	19,876.1	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	19,876.1	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	5,773.3	5,841.5
Investment debt securities held-to-maturity	2,497.8	2,874.0
Accrued interest receivable	125.5	129.1
Mortgage servicing rights, net	40.0	38.8
Loans held for sale	3.2	0.5
Net loans held for investment	17,241.0	17,281.8
Derivative assets	44.2	37.4
Deferred compensation plan assets	19.9	19.2
Total financial assets	25,744.9	26,222.3
Financial liabilities:
Total deposits, excluding time deposits	0.0	0.0
Time deposits	2,906.3	2,981.7
Securities sold under repurchase agreements	794.2	782.7
Other Borrowings	2,342.0	2,603.0
Accrued interest payable	53.6	52.2
Long-term debt	367.9	115.6
Subordinated debentures held by subsidiary trusts	154.6	151.1
Derivative liabilities	156.6	127.3
Deferred compensation plan liabilities	19.9	19.2
Total financial liabilities	6,795.1	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	19.5	46.9
Net loans held for investment	116.4	52.6
Derivative assets	0.0	0.0
Deferred compensation plan assets	0.0	0.0
Total financial assets	135.9	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