Securities Sold Under Agreements to Repurchase - Additional Information (Detail)	9 Months Ended
Sep. 30, 2020
Securities Sold under Agreements to Repurchase [Abstract]
Gross outstanding balance of repurchase agreements collateralized by securities percentage	106.00%

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:	num:percentItemType
Balance Type:	na
Period Type:	duration