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v3.21.1

Financial Guarantees (Detail) - USD ($) | 3 Months Ended |
--- | --- | ---
Mar. 31, 2021 | Dec. 31, 2020
Financial Guarantees [Line Items] |  |
Term of standby letters of credit, years | 1 year |
Notional amount of standby letters of credit | $ 75,100,000 | $ 86,900,000
Carrying value of letters of credit | 500,000 | $ 500,000
Interest Rate Swap |  |
Financial Guarantees [Line Items] |  |
Notional amount | $ 68,200,000 |

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- 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

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- Definition Financial guarantees. + References No definition available. + Details
Name: | onb_FinancialGuaranteesLineItems
Namespace Prefix: | onb_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- 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

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- Definition Length of time that standby letters of credit are effective. + References No definition available. + Details
Name: | onb_TermOfStandbyLettersOfCredit
Namespace Prefix: | onb_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- 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 http://asc.fasb.org/extlink&oid=121590274&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 http://asc.fasb.org/extlink&oid=121590274&loc=SL5579245-113959 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | instant

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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |