Investment Securities - Schedule of Proceeds from Sales or Calls and Realized Gain and Losses of Available-for-sale Investment Securities and Other Securities (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sales of available-for-sale securities	$ 170,265	$ 16,523
Proceeds from calls of available-for-sale securities	132,471	91,335
Total	221,859	39,898
Realized gains on sales of available-for-sale securities	2,481	658
Realized losses on sales of available-for-sale securities	(25)
Other securities gains	62onb_OtherSecuritiesGains	168onb_OtherSecuritiesGains
Net securities gains	2,683us-gaap_GainLossOnSaleOfSecuritiesNet	559us-gaap_GainLossOnSaleOfSecuritiesNet
Calls [Member]
Schedule of Available-for-sale Securities [Line Items]
Proceeds from calls of available-for-sale securities	51,594	23,375
Realized gains on calls of available-for-sale securities	168
Realized losses on calls of available-for-sale securities	$ (3)	$ (267)

Investment Securities - Schedule of Proceeds from Sales or Calls and Realized Gain and Losses of Available-for-sale Investment Securities and Other Securities (Detail) (USD $)
In Thousands, unless otherwise specified

3 Months Ended

Mar. 31, 2015

Mar. 31, 2014

Schedule of Available-for-sale Securities [Line Items]

Proceeds from sales of available-for-sale securities

$ 170,265us-gaap_ProceedsFromSaleOfAvailableForSaleSecurities

$ 16,523us-gaap_ProceedsFromSaleOfAvailableForSaleSecurities

Proceeds from calls of available-for-sale securities

132,471us-gaap_ProceedsFromMaturitiesPrepaymentsAndCallsOfAvailableForSaleSecurities

91,335us-gaap_ProceedsFromMaturitiesPrepaymentsAndCallsOfAvailableForSaleSecurities

Total

221,859us-gaap_ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities

39,898us-gaap_ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities

Realized gains on sales of available-for-sale securities

2,481us-gaap_AvailableForSaleSecuritiesGrossRealizedGains

658us-gaap_AvailableForSaleSecuritiesGrossRealizedGains

Realized losses on sales of available-for-sale securities

(25)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses

Other securities gains

62onb_OtherSecuritiesGains

168onb_OtherSecuritiesGains

Net securities gains

2,683us-gaap_GainLossOnSaleOfSecuritiesNet

559us-gaap_GainLossOnSaleOfSecuritiesNet

Calls [Member]

Schedule of Available-for-sale Securities [Line Items]

Proceeds from calls of available-for-sale securities

51,594us-gaap_ProceedsFromMaturitiesPrepaymentsAndCallsOfAvailableForSaleSecurities
/ us-gaap_InformationByCategoryOfDebtSecurityAxis
= onb_CallsByIssuersMember

23,375us-gaap_ProceedsFromMaturitiesPrepaymentsAndCallsOfAvailableForSaleSecurities
/ us-gaap_InformationByCategoryOfDebtSecurityAxis
= onb_CallsByIssuersMember

Realized gains on calls of available-for-sale securities

168onb_RealizedGainsOnCallsOfAvailableForSaleSecurities
/ us-gaap_InformationByCategoryOfDebtSecurityAxis
= onb_CallsByIssuersMember

Realized losses on calls of available-for-sale securities

$ (3)onb_RealizedLossOnCallsOfAvailableForSaleSecurities
/ us-gaap_InformationByCategoryOfDebtSecurityAxis
= onb_CallsByIssuersMember

$ (267)onb_RealizedLossOnCallsOfAvailableForSaleSecurities
/ us-gaap_InformationByCategoryOfDebtSecurityAxis
= onb_CallsByIssuersMember

- Definition

Represents the gain or loss realized on the sale of trading securities or mutual funds.

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No definition available.

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- Definition

The gross profit realized on the call of available-for-sale debt or equity securities.

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- Definition

The gross loss realized on the call of available-for-sale debt or equity securities.

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No definition available.

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- Definition

This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 9

-Subparagraph (a)

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- Definition

The net gain (loss) realized from the sale, exchange, redemption, or retirement of securities, not separately or otherwise categorized as trading, available-for-sale, or held-to-maturity.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.13(h))

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 13

-Subparagraph h

-Article 9

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.3)

-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 3

-Article 7

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- Definition

The cash inflow associated with maturities (principal being due), prepayments and calls (requests of early payments) on securities not classified as either held-to-maturity securities or trading securities which are classified as available-for-sale securities.

+ 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

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 11

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26853-111562

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 12

-Subparagraph (a),(b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3179-108585

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The cash inflow associated with the sale or maturity (principal being due) of securities not classified as either held-to-maturity securities or trading securities which are classified as available-for-sale securities.

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The cash inflow associated with the sale of debt and equity securities classified as available-for-sale securities.

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