Qualified Affordable Housing Projects and Other Tax Credit Investments (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021
Investment Holdings [Line Items]
Investment	$ 119,970		$ 110,942
Unfunded Commitment	53,626		56,607
Amortization Expense	2,769	$ 2,064
Tax Expense (Benefit) Recognized	(3,276)	(2,843)
LIHTC
Investment Holdings [Line Items]
Investment, Proportional amortization	67,703		68,989
Unfunded Commitment, Proportional amortization	39,830		41,355
Amortization Expense	1,253	862
Tax Expense (Benefit) Recognized	(1,650)	(1,136)
FHTC
Investment Holdings [Line Items]
Investment, Equity	21,036		21,241
Unfunded Commitment, Equity	13,796		15,252
Amortization Expense	205	130
Tax Expense (Benefit) Recognized	(251)	(682)
NMTC
Investment Holdings [Line Items]
Investment, Equity	29,455		18,727
Unfunded Commitment, Equity	0		0
Amortization Expense	1,101	375
Tax Expense (Benefit) Recognized	(1,375)	(463)
Renewable Energy
Investment Holdings [Line Items]
Investment, Equity	1,776		1,985
Unfunded Commitment, Equity	0		$ 0
Amortization Expense	210	697
Tax Expense (Benefit) Recognized	$ 0	$ (562)

Amortization method qualified affordable housing project investments commitment.

No definition available.

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

- Definition

Equity method investment qualified affordable housing project investments.

+ References

No definition available.

+ Details

Name:	onb_EquityMethodInvestmentQualifiedAffordableHousingProjectInvestments
Namespace Prefix:	onb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Equity method qualified affordable housing project investments commitment.

+ References

No definition available.

+ Details

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

- Definition

Investments in qualified affordable housing projects.

+ References

No definition available.

+ Details

Name:	onb_InvestmentsInQualifiedAffordableHousingProjects
Namespace Prefix:	onb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of affordable housing tax credits and other tax benefits recognized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 740
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=116633289&loc=SL49122993-202985

+ Details

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

- Definition

Amount of investment in qualified affordable housing projects.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 740
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=116633289&loc=SL49122993-202985

+ Details

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

- Definition

Amount of amortization expense of investment in qualified affordable housing projects.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 740
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=116633289&loc=SL49122993-202985

+ Details

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

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

- Definition

Amount of commitment or contingent commitments to qualified affordable housing project investments.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 740
-Section 50
-Paragraph 2
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=116633289&loc=SL49122993-202985

+ Details

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

- Details

Name:	us-gaap_InvestmentTypeAxis=onb_LowIncomeHousingTaxCreditsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=onb_FederalHistoricTaxCreditsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=onb_NewMarketsTaxCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_RenewableEnergyProgramMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: