Commercial real estate loans as a percentage of risk-based capital.

No definition available.

Name:	onb_CommercialRealEstateLoansAsPercentageOfRiskBasedCapital
Namespace Prefix:	onb_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The regulatory guideline limit for commercial real estate loans as a percentage of risk-based capital.

+ References

No definition available.

+ Details

Name:	onb_CommercialRealEstateLoansAsPercentageOfRiskBasedCapitalRegulatoryGuideLineLimit
Namespace Prefix:	onb_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Maximum number of days past due before loan is placed on nonaccrual status.

+ References

No definition available.

+ Details

Name:	onb_FinanceReceivablePastDuePeriodForNonaccrualStatus
Namespace Prefix:	onb_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Financing Receivable, Number Of Loan Portfolios

+ References

No definition available.

+ Details

Name:	onb_FinancingReceivableNumberOfLoanPortfolios
Namespace Prefix:	onb_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Financing Receivable, Number Of Loan Segments

+ References

No definition available.

+ Details

Name:	onb_FinancingReceivableNumberOfLoanSegments
Namespace Prefix:	onb_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of loan participations with other financial institutions.

+ References

No definition available.

+ Details

Name:	onb_LoanParticipations
Namespace Prefix:	onb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of loan participations retained.

+ References

No definition available.

+ Details

Name:	onb_LoanParticipationsRetained
Namespace Prefix:	onb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of loan participations sold to other financial institutions.

+ References

No definition available.

+ Details

Name:	onb_LoanParticipationsSold
Namespace Prefix:	onb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Loans and Leases Receivable, Related Parties, Percent Of Total Equity

+ References

No definition available.

+ Details

Name:	onb_LoansAndLeasesReceivableRelatedPartiesPercentOfTotalEquity
Namespace Prefix:	onb_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount, before allowance for credit loss, of accrued interest on financing receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 3B
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-3B

+ Details

Name:	us-gaap_FinancingReceivableAccruedInterestBeforeAllowanceForCreditLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph c
-Publisher SEC

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, excluding accrued interest, of credit loss expense (reversal of expense) on financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestCreditLossExpenseReversal
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amortized cost of financing receivable with payment default in current period and modified within previous 12 months preceding payment default for debtor experiencing financial difficulty.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 34
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-34

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 12A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481933/310-10-55-12A

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 44
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-44

+ Details

Name:	us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of allowance for credit loss at acquisition date of financing receivable purchased with credit deterioration.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 19
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-19

+ Details

Name:	us-gaap_FinancingReceivablePurchasedWithCreditDeteriorationAllowanceForCreditLossAtAcquisitionDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of commitment to lend additional funds to debtor experiencing financial difficulty for which creditor has modified terms of financing receivable.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481746/310-40-50-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 36
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-36

+ Details

Name:	us-gaap_LoansAndLeasesReceivableImpairedCommitmentToLend
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_BusinessAcquisitionAxis=onb_CapStarFinancialHoldingsIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Loans and Allowance for Credit Losses - Additional Information (Details)		12 Months Ended
	Apr. 01, 2024 USD ($)	Dec. 31, 2024 USD ($) portfolio segment	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan participations		$ 3,300,000,000
Loan participations sold		1,500,000,000
Loan participations retained		$ 1,800,000,000
Number of loan portfolios \| portfolio		4
Number of loan segments \| segment		7
Allowance for credit losses at acquisition	$ 26,700,000	$ 26,725,000
Provision for credit losses		110,619,000	$ 58,887,000	$ 144,799,000
Accrued interest receivable on loans		$ 171,600,000	169,800,000
Loan placed on nonaccrual when past due, number of days		90 days
Financing receivable, modified, subsequent default		$ 8,800,000	$ 0
Unfunded commitments on TDRs		0
CapStar Financial Holdings, Inc.
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses at acquisition	$ 26,700,000
Provision for credit losses		$ 15,300,000
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Percentage of risk-based capital		237.00%
Regulatory guideline limit		300.00%
Maximum
Financing Receivable, Allowance for Credit Losses [Line Items]
Aggregate amount of loans to related parties		5.00%	5.00%