Federal Home Loan Bank Advances - Additional Information (Details) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Federal Home Loan Bank, Advances [Line Items]
Weighted-average rates of FHLB advances	3.45%	3.15%
Modifications, unamortized prepayment fees	$ 18.7	$ 20.2
FHLB Advances
Federal Home Loan Bank, Advances [Line Items]
Percentage of borrowings collateralized by investment securities and residential real estate loans	148.00%

Advances From Federal Home Loan Banks, Modifications, Unamortized Prepayment Fees

No definition available.

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

- Definition

Amount of collateral pledged as a percentage of outstanding secured debt.

+ References

No definition available.

+ Details

Name:	onb_PercentageOfBorrowingsCollateralizedByInvestmentSecuritiesAndResidentialRealEstateLoans
Namespace Prefix:	onb_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The calculated weighted average interest rate for Federal Home Loan Bank advances outstanding as of the Balance Sheet date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.13(3)(a))
-URI https://asc.fasb.org/extlink&oid=126897435&loc=d3e534808-122878

+ Details

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

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_FederalHomeLoanBankAdvancesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: