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Securities Sold Under Agreements to Repurchase - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2020
Securities Sold under Agreements to Repurchase [Abstract]
Gross outstanding balance of repurchase agreements collateralized by securities percentage	114.00%

X

- 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