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Securities Sold Under Agreements to Repurchase - Additional Information (Detail) | 3 Months Ended
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Mar. 31, 2021
Securities Sold under Agreements to Repurchase [Abstract] |
Gross outstanding balance of repurchase agreements collateralized by securities percentage | 106.00%

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- Definition Fair value of securities pledged as security as a percentage of the gross outstanding balance of repurchase agreements. + References No definition available. + Details
Name: | onb_PercentagePledgedSecuritiesOfRepurchaseAgreementsOutstandingBalance
Namespace Prefix: | onb_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration