Securities Sold Under Agreements to Repurchase - Additional Information (Details)	3 Months Ended
Mar. 31, 2022
Securities Sold under Agreements to Repurchase [Abstract]
Gross outstanding balance of repurchase agreements collateralized by securities percentage	116.00%

Fair value of securities pledged as security as a percentage of the gross outstanding balance of repurchase agreements.

No definition available.

Name:	onb_PercentagePledgedSecuritiesOfRepurchaseAgreementsOutstandingBalance
Namespace Prefix:	onb_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration