Commitments, Contingencies, and Financial Guarantees - Loan Commitments and Standby Letters of Credit (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Commitments and Contingencies Disclosure [Abstract]
Unfunded loan commitments	$ 8,912,587	$ 8,979,334
Standby letters of credit	192,237	174,070
Carrying value of letters of credit	$ 1,300	$ 800

Amount of agreement to extend credit in the form of a standby letter of credit.

No definition available.

Name:	onb_NotionalAmountOfStandbyLettersOfCredit
Namespace Prefix:	onb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant