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Commitments, Contingencies, and Financial Guarantees - Loan Commitments and Standby Letters of Credit (Details) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024
Commitments and Contingencies Disclosure [Abstract]
Unfunded loan commitments	$ 12,145,320	$ 8,533,433
Standby letters of credit	199,638	194,323
Cancellable loan commitments	2,800,000	2,500,000
Carrying value of letters of credit	$ 1,700	$ 1,700

X

- 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

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