Derivative Financial Instruments - Additional Information (Details) - USD ($) $ in Thousands	Sep. 30, 2022	Dec. 31, 2021
Derivative [Line Items]
Percentage of periodic changes in fair value qualifies for hedge accounting treatment	100.00%
Interest Income
Derivative [Line Items]
Reclassified interest income (expense)	$ 3,900
Interest Expense
Derivative [Line Items]
Reclassified interest income (expense)	15,700
Interest rate swaps on borrowings \| Cash flow hedges \| Designated as Hedging Instrument
Derivative [Line Items]
Notional amount	150,000	$ 150,000
Interest rate swaps on borrowings \| Fair value hedges \| Designated as Hedging Instrument
Derivative [Line Items]
Notional amount	327,500	377,500
Interest rate collars and floors on loan pools \| Cash flow hedges \| Designated as Hedging Instrument
Derivative [Line Items]
Notional amount	1,500,000	600,000
Interest rate swaps on investment securities \| Fair value hedges \| Designated as Hedging Instrument
Derivative [Line Items]
Notional amount	909,957	909,957
Interest rate lock commitments \| Not Designated as Hedging Instrument
Derivative [Line Items]
Notional amount	51,177	90,731
Forward mortgage loan contracts \| Not Designated as Hedging Instrument
Derivative [Line Items]
Notional amount	71,000	126,107
Counterparty interest rate swaps \| Not Designated as Hedging Instrument
Derivative [Line Items]
Notional amount	5,062,757	2,433,177
Customer interest rate swaps \| Not Designated as Hedging Instrument
Derivative [Line Items]
Notional amount	$ 5,062,757	$ 2,433,177

Percentage of periodic changes in fair value qualifies for hedge accounting.

No definition available.

Name:	onb_PercentageOfPeriodicChangesInFairValueQualifiesForHedgeAccounting
Namespace Prefix:	onb_
Data Type:	dtr-types:percentItemType
Balance Type:	na
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

No definition available.

+ Details

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

- Definition

Nominal or face amount used to calculate payment on derivative.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI https://asc.fasb.org/extlink&oid=125515794&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI https://asc.fasb.org/extlink&oid=125515794&loc=SL5579245-113959

+ Details

Name:	us-gaap_DerivativeNotionalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

The estimated net amount of unrealized gains or losses on interest rate cash flow hedges as of the balance sheet date expected to be reclassified to earnings within the next twelve months.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI https://asc.fasb.org/extlink&oid=121549185&loc=d3e80748-113994

+ Details

Name:	us-gaap_InterestRateCashFlowHedgeGainLossToBeReclassifiedDuringNext12MonthsNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_IncomeStatementLocationAxis=us-gaap_InterestIncomeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementLocationAxis=us-gaap_InterestExpenseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=onb_VariableInterestRateSwapBorrowingsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_CashFlowHedgingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_DesignatedAsHedgingInstrumentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_FairValueHedgingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=onb_VariableInterestRateCollarsAndFloorsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=onb_VariableInterestRateSwapAvailableForSaleInvestmentSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateLockCommitmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_NondesignatedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=onb_ForwardCommitmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=onb_OffsettingCounterPartyDerivativeInstrumentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=onb_CustomerDerivativeInstrumentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: