Other Borrowings - Summary of Other Borrowings (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Debt Instrument [Line Items]
Lease liability	$ 20,108	$ 20,955
Other borrowings	813,213	764,870
Old National Bank
Debt Instrument [Line Items]
Other basis adjustments	134,370	97,872
Lease liability	20,108	20,955
Derivative collateralize position amount	134,200	97,600
Old National Bancorp
Debt Instrument [Line Items]
Other basis adjustments	$ 16,916	18,207
Senior Unsecured Notes | Old National Bancorp
Debt Instrument [Line Items]
Fixed rates	4.125%
Senior unsecured notes (fixed rate 4.125%) maturing August 2024	$ 175,000	175,000
Unamortized debt issuance costs related to senior unsecured notes	$ (52)	(91)
Subordinated Debt | Old National Bank
Debt Instrument [Line Items]
Variable rates	9.94%
Subordinated debentures	$ 12,000	12,000
Subordinated Debt | Old National Bank | Secured Overnight Financing Rate
Debt Instrument [Line Items]
Variable rate	4.618%
Subordinated Debt | Old National Bancorp
Debt Instrument [Line Items]
Fixed rates	5.875%
Subordinated debentures	$ 150,000	150,000
Junior Subordinated Debentures | Old National Bancorp
Debt Instrument [Line Items]
Junior subordinated debentures (rates of 6.95% to 9.15%) maturing July 2031 to September 2037	136,643	136,643
Leveraged Loans for NMTC | Old National Bank
Debt Instrument [Line Items]
Leveraged loans for NMTC (fixed rates of 1.00% to 1.43%) maturing December 2046 to June 2060	$ 168,228	$ 154,284
Minimum | Junior Subordinated Debentures | Old National Bancorp
Debt Instrument [Line Items]
Variable rates	6.95%
Minimum | Leveraged Loans for NMTC | Old National Bank
Debt Instrument [Line Items]
Fixed rates	1.00%
Maximum | Junior Subordinated Debentures | Old National Bancorp
Debt Instrument [Line Items]
Variable rates	9.15%
Maximum | Leveraged Loans for NMTC | Old National Bank
Debt Instrument [Line Items]
Fixed rates	1.43%