Deposits to be included in transaction.

No definition available.

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- Definition

Number of banking centers.

+ References

No definition available.

+ Details

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- Definition

Number of branches.

+ References

No definition available.

+ Details

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Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of banking centers consolidated into existing branch locations.

+ References

No definition available.

+ Details

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- 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=26713463&loc=d3e16265-109275

+ Details

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Balance Type:	na
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- Definition

The carrying amount of assets an investment adviser manages on behalf of investors.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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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Balance Type:	credit
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- Definition

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

+ References

+ Details

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- Details

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Balance Type:	na
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- 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=34752592&loc=d3e1043-128460

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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=39739317&loc=d3e4845-128472

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- Definition

Gross carrying amount before accumulated amortization as of the balance sheet date to an asset acquired in a business combination representing a favorable existing relationship with customers having a finite beneficial life.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 30

-Section 50

-Paragraph 2

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16323-109275

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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, resulting from a business combination.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	debit
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- Definition

Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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Balance Type:	na
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- Definition

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

+ 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 (a)

-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The face amount of loans that will be sold to other entities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=28367877&loc=d3e4428-111522

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- 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

-Glossary Investing Activities

-URI http://asc.fasb.org/extlink&oid=6516133

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 13

-Subparagraph (b)

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Acquisition and Divestiture Activity - Additional Information (Detail) (USD $) Share data in Millions, unless otherwise specified	3 Months Ended	0 Months Ended
	Mar. 31, 2015 Branch	Jan. 30, 2015 Branches	Nov. 01, 2014 BankingCenters	Feb. 01, 2015	Apr. 25, 2014 Branch	Jun. 30, 2014	Jul. 31, 2014 Branch	Jan. 01, 2015 Branch
Business Acquisition [Line Items]
Goodwill acquired during the period	$ 57,059,000us-gaap_GoodwillAcquiredDuringPeriod
Loans held for sale	186,200,000
Deposits to be included in transaction	620,200,000onb_DepositsToBeIncludedInTransaction
Number of banking centers consolidated	19onb_NumberOfLocationsToBeConsolidated
Insurance [Member]
Business Acquisition [Line Items]
Goodwill acquired during the period	1,090,000
Southern Illinois Franchise [Member]
Business Acquisition [Line Items]
Number of banking centers		12
Eastern Indiana [Member]
Business Acquisition [Line Items]
Number of banking centers		4
Ohio [Member]
Business Acquisition [Line Items]
Number of banking centers		1
Tower Financial Corporation [Member]
Business Acquisition [Line Items]
Additional assets under management					556,000,000
Number of branch locations					7
Total purchase price of acquisition	110,400,000
Purchase price of acquisition, cash	31,700,000
Issuance of common stock shares for acquisitions of business	5.6
Options and stock appreciation rights, value	78,700,000
Transaction and integration costs associated with the acquisition	5,600,000
Tower Financial Corporation [Member] \| Core Deposit Intangible [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired identifiable intangible assets	7 years
Definite-lived intangible assets acquired	4,600,000
United Bancorp, Inc. [Member]
Business Acquisition [Line Items]
Additional assets under management						688,000,000
Total purchase price of acquisition	157,800,000
Purchase price of acquisition, cash	34,000,000
Issuance of common stock shares for acquisitions of business	9.1
Transaction and integration costs associated with the acquisition	7,300,000
United Bancorp, Inc. [Member] \| Core Deposit Intangible [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired identifiable intangible assets	7 years
Definite-lived intangible assets acquired	5,900,000
United Bancorp, Inc. [Member] \| Common Stock [Member]
Business Acquisition [Line Items]
Options and stock appreciation rights, value	122,000,000
United Bancorp, Inc. [Member] \| Options and Stock Appreciation Rights [Member]
Business Acquisition [Line Items]
Options and stock appreciation rights, value	1,800,000
United Bancorp, Inc. [Member] \| Michigan [Member]
Business Acquisition [Line Items]
Number of branch locations						18	36
LSB Financial Corp. [Member]
Business Acquisition [Line Items]
Total purchase price of acquisition	69,600,000
Purchase price of acquisition, cash	17,800,000
Issuance of common stock shares for acquisitions of business	3.6
Transaction and integration costs associated with the acquisition	2,900,000
Number of banking centers			5
LSB Financial Corp. [Member] \| Core Deposit Intangible [Member]
Business Acquisition [Line Items]
Definite-lived intangible assets acquired	2,600,000
LSB Financial Corp. [Member] \| Community Banking [Member] \| Core Deposit Intangible [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired identifiable intangible assets	7 years
LSB Financial Corp. [Member] \| Common Stock [Member]
Business Acquisition [Line Items]
Options and stock appreciation rights, value	51,800,000
Founders Financial Corporation [Member]
Business Acquisition [Line Items]
Number of branch locations								4
Total purchase price of acquisition	91,700,000
Purchase price of acquisition, cash	41,000,000
Issuance of common stock shares for acquisitions of business	3.4
Options and stock appreciation rights, value	50,600,000
Transaction and integration costs associated with the acquisition	3,700,000
Founders Financial Corporation [Member] \| Core Deposit Intangible [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired identifiable intangible assets	7 years
Definite-lived intangible assets acquired	3,000,000
Mutual Underwriters [Member]
Business Acquisition [Line Items]
Total purchase price of acquisition				3,700,000
Mutual Underwriters [Member] \| Customer Business Relationships [Member]
Business Acquisition [Line Items]
Estimated useful life of intangible assets	10 years
Mutual Underwriters [Member] \| Insurance [Member]
Business Acquisition [Line Items]
Goodwill acquired during the period				1,100,000
Mutual Underwriters [Member] \| Insurance [Member] \| Customer Business Relationships [Member]
Business Acquisition [Line Items]
Customer business relationship intangibles acquired				$ 2,600,000