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Fair Value - Reconciliation of All Assets Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Level 3) (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Accretion/(amortization) of discount or premium $ 5onb_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAccretionAmortizationOfDiscountOrPremium $ 5onb_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAccretionAmortizationOfDiscountOrPremium
Significant Unobservable Inputs (Level 3) [Member] | Pooled Trust Preferred Securities Available-for-Sale [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning balance 6,607us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= onb_PooledTrustPreferredSecuritiesAvailableForSaleMember
8,037us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= onb_PooledTrustPreferredSecuritiesAvailableForSaleMember
Accretion/(amortization) of discount or premium 5onb_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAccretionAmortizationOfDiscountOrPremium
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= onb_PooledTrustPreferredSecuritiesAvailableForSaleMember
4onb_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAccretionAmortizationOfDiscountOrPremium
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= onb_PooledTrustPreferredSecuritiesAvailableForSaleMember
Payments received (264)onb_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPaymentsReceived
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= onb_PooledTrustPreferredSecuritiesAvailableForSaleMember
(963)onb_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPaymentsReceived
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= onb_PooledTrustPreferredSecuritiesAvailableForSaleMember
Increase/(decrease) in fair value of securities 205us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPeriodIncreaseDecrease
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= onb_PooledTrustPreferredSecuritiesAvailableForSaleMember
(299)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPeriodIncreaseDecrease
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= onb_PooledTrustPreferredSecuritiesAvailableForSaleMember
Ending balance 6,553us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= onb_PooledTrustPreferredSecuritiesAvailableForSaleMember
6,779us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= onb_PooledTrustPreferredSecuritiesAvailableForSaleMember
Significant Unobservable Inputs (Level 3) [Member] | States and Political Subdivisions [Member]    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning balance 325us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
669us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Accretion/(amortization) of discount or premium   1onb_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAccretionAmortizationOfDiscountOrPremium
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Matured securities (325)onb_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetMaturedSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
(326)onb_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetMaturedSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Ending balance   $ 344us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember