Business acquisition percentage of voting interests acquired receipt of cash.

No definition available.

Name:	onb_BusinessAcquisitionPercentageOfVotingInterestsAcquiredReceiptOfCash
Namespace Prefix:	onb_
Data Type:	num:percentItemType
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Period Type:	duration

- Definition

Business combination cash consideration, per share amount.

+ References

No definition available.

+ Details

Name:	onb_BusinessCombinationCashConsiderationPerShareAmount
Namespace Prefix:	onb_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Business combination equity interest exchange ratio.

+ References

No definition available.

+ Details

Name:	onb_BusinessCombinationEquityInterestExchangeRatio
Namespace Prefix:	onb_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of gain (loss) before income tax on sale or disposal of assets utilized in financial service operations.

+ References

No definition available.

+ Details

Name:	onb_GainsLossesOnSalesOfAssetsBeforeIncomeTax
Namespace Prefix:	onb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Number of banking centers associated with transaction.

+ References

No definition available.

+ Details

Name:	onb_NumberOfBankingCenters
Namespace Prefix:	onb_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of banking centers consolidated into existing branch locations.

+ References

No definition available.

+ Details

Name:	onb_NumberOfLocationsToBeConsolidated
Namespace Prefix:	onb_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Weighted average amortization period of finite-lived intangible assets acquired either individually or as part of a group of assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16265-109275

+ Details

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

- Definition

Value of equity interests (such as common shares, preferred shares, or partnership interest) issued or issuable to acquire the entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (b)(4)
-URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479

+ Details

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

- Definition

Number of shares of equity interests issued or issuable to acquire entity.

+ References

+ Details

Name:	us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
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_BusinessAcquisitionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

This element represents acquisition-related costs incurred to effect a business combination which costs have been expensed during the period. Such costs include finder's fees; advisory, legal, accounting, valuation, and other professional or consulting fees; general administrative costs, including the costs of maintaining an internal acquisitions department; and may include costs of registering and issuing debt and equity securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 25
-Paragraph 23
-URI http://asc.fasb.org/extlink&oid=84229725&loc=d3e1043-128460

+ Details

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

- Definition

Amount recognized for assets, including goodwill, in excess of (less than) the aggregate liabilities assumed.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84161108&loc=d3e4845-128472

+ Details

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

- Definition

Amount of increase in assets, excluding financial assets, lacking physical substance with a definite life, from an acquisition.

+ References

No definition available.

+ Details

Name:	us-gaap_FinitelivedIntangibleAssetsAcquired1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of gain (loss) on sale or disposal of assets utilized in financial service operations.

+ References

No definition available.

+ Details

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

- Definition

Amount of divestiture of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

+ Details

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

- Definition

The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 13
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3213-108585

+ Details

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

- Definition

This element represents the cash inflow during the period from the sale of a component of the entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3179-108585

+ Details

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

- Details

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Data Type:	na
Balance Type:
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- Details

Name:	us-gaap_BusinessAcquisitionAxis=onb_AnchorMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Data Type:	na
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- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementGeographicalAxis=onb_MadisonMilwaukeeAndFoxValleyTriangleMember
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Acquisition and Divestiture Activity - Additional Information (Detail) $ / shares in Units, $ in Thousands, shares in Millions		1 Months Ended		6 Months Ended		12 Months Ended
	May 01, 2016 BankingCenters	Jan. 31, 2017 Branch	May 31, 2016 USD ($)	Jun. 30, 2017 USD ($) $ / shares shares	Jun. 30, 2016 USD ($)	Dec. 31, 2016 Branch
Business Acquisition [Line Items]
Cash received in acquisition					$ 91,771
Goodwill and intangible assets eliminated				$ 0	$ 40,963
Number of banking centers consolidated \| Branch		15				5
ONB Insurance Group, Inc. [Member]
Business Acquisition [Line Items]
Cash received in acquisition			$ 91,800
Gain on sale of assets before tax			41,900
Gain on sale of assets after tax			17,600
Goodwill and intangible assets eliminated			$ 47,500
Anchor BanCorp Wisconsin Inc [Member]
Business Acquisition [Line Items]
Number of banking centers \| BankingCenters	46
Portion of share received by merged entity				3.5505
Consideration received in cash by merged entity \| $ / shares				$ 48.50
Total fair value of consideration paid				$ 459,800
Purchase price of acquisition, cash				$ 186,200
Issuance of common stock shares for acquisitions of business \| shares				20.4
Value of issuance common stock shares				$ 273,600
Transaction and integration costs associated with the acquisition				$ 15,900
Anchor BanCorp Wisconsin Inc [Member] \| Maximum [Member]
Business Acquisition [Line Items]
Maximum percentage of purchase price in cash				40.00%
Anchor BanCorp Wisconsin Inc [Member] \| Core Deposit [Member]
Business Acquisition [Line Items]
Estimated useful life of intangible assets				7 years
Estimated fair value of intangible assets acquired				$ 21,600
Anchor BanCorp Wisconsin Inc [Member] \| Madison Milwaukee and Fox Valley Triangle [Member]
Business Acquisition [Line Items]
Number of banking centers \| BankingCenters	32