Derivative Financial Instruments - Summary of collars designated as cash flow hedges (Detail) - USD ($)	9 Months Ended	12 Months Ended
Sep. 30, 2019	Dec. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount	$ 567,000,000.0	$ 1,482,000,000
Variable Interest Rate Collar [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount	$ 300,000,000	$ 200,000,000
Weighted average cap rates	3.21%	3.44%
Weighted average floor rates	2.21%	2.38%
Weighted average rates	2.08%	2.35%
Weighted average maturity (in years)	2 years 1 month 6 days	2 years 9 months 18 days
Unrealized gains (losses)	$ 4,852,000	$ 1,309,000

Derivative average interest rate.

No definition available.

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

- Definition

Average cap rate on a group of interest rate derivatives, such as interest rate caps or collars. If market rates exceed the cap rate, a payment or receipt is triggered on the contract.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeAverageCapInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Average floor rate on a group of interest rate derivatives, such as interest rate floor or collars. If market rates fall below the floor rate, a payment or receipt is triggered on the contract.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeAverageFloorInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Average remaining period until maturity of the derivative contract, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeAverageRemainingMaturity1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
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_DerivativeInstrumentsGainLossLineItems
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 http://asc.fasb.org/extlink&oid=109980867&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=109980867&loc=SL5579245-113959

+ Details

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

- Definition

The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585

+ Details

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

- Details

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