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Commitments, Contingencies, and Financial Guarantees - Loan Commitments and Standby Letters of Credit (Details) - USD ($) $ in Thousands | Sep. 30, 2025 | Dec. 31, 2024
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Commitments and Contingencies Disclosure [Abstract] |  |
Unfunded loan commitments | $ 11,741,820 | $ 8,533,433
Standby letters of credit | 266,892 | 194,323
Cancellable loan commitments | 2,900,000 | 2,500,000
Carrying value of letters of credit | $ 1,600 | $ 1,700

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- Definition Cancellable Loan Commitments + References No definition available. + Details
Name: | onb_CancellableLoanCommitments
Namespace Prefix: | onb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- 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