Loan Servicing Rights - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Transfers And Servicing [Abstract]
Loan servicing rights	$ 25,368	$ 24,497	$ 24,661
Principal balance of loans serviced for others	3,445,000	3,306,000
Funds held in escrow	$ 12,700	10,700
Percentage of mortgage loan	99.80%
Fair value of servicing rights	$ 26,500	$ 27,400
Fair value at discount rate	12.00%	12.00%
Fair value inputs weighted average prepayment speed	167.00%	119.00%

Fair value inputs discount rate 1.

No definition available.

Name:	onb_FairValueInputsDiscountRate1
Namespace Prefix:	onb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Weighted average prepayment speed percentage used to find the present value of an amount to be received in the future as an input to measure fair value.

+ References

No definition available.

+ Details

Name:	onb_FairValueInputsWeightedAveragePrepaymentSpeed
Namespace Prefix:	onb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of loan servicing rights that are mortgage loans.

+ References

No definition available.

+ Details

Name:	onb_PercentageOfMortgageLoans
Namespace Prefix:	onb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The principal balance of the loans serviced for others related to the Company's loan servicing rights.

+ References

No definition available.

+ Details

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

- Definition

The designation of funds furnished by a borrower to a lender to assure future payments of the borrower's real estate taxes and insurance obligations with respect to a mortgaged property. Escrow deposits may be made for a variety of other purposes such as earnest money and contingent payments. This element excludes replacement reserves which are an escrow separately provided for within the US GAAP taxonomy.

+ 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 210.9-03.10)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

Aggregate amount of servicing assets that are subsequently measured at fair value and servicing assets that are subsequently measured using the amortization method.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6469459&loc=d3e122501-111745

+ Details

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

- Definition

Fair value of an asset representing net future revenue from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6469459&loc=d3e122501-111745

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=116651436&loc=d3e122625-111746

+ Details

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

- References

No definition available.

+ Details

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