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Commitments, Contingencies, and Financial Guarantees - Loan Commitments and Standby Letters of Credit (Details) - USD ($) $ in Thousands | Sep. 30, 2023 | Dec. 31, 2022
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Commitments and Contingencies Disclosure [Abstract] |  |
Unfunded loan commitments | $ 9,142,538 | $ 8,979,334
Standby letters of credit | 184,027 | 174,070
Carrying value of letters of credit | $ 1,200 | $ 800

X
- Definition Carrying value of standby letters of credit financial instruments. + References No definition available. + Details
Name: | onb_CarryingValueOfLetterOfCredit
Namespace Prefix: | onb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of agreement to extend credit in the form of a loan. + References No definition available. + Details
Name: | onb_LoanCommitments
Namespace Prefix: | onb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of agreement to extend credit in the form of a standby letter of credit. + References No definition available. + Details
Name: | onb_NotionalAmountOfStandbyLettersOfCredit
Namespace Prefix: | onb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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