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Covered Loans - Additional Information (Detail) (USD $)
 3 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Mar. 31, 2014
Dec. 31, 2013
Covered Loans [Line Items]        
Covered loans $ 136,840,000us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCovered $ 147,708,000us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCovered $ 194,161,000us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCovered  
Allowance for loan losses related to the acquired loans 0us-gaap_AllowanceForLoanAndLeaseLossesLoansAcquired      
Outstanding covered loans including principal, interest, fees and penalties 44,167,000us-gaap_FinancingReceivableAcquiredWithDeterioratedCreditQuality 47,370,000us-gaap_FinancingReceivableAcquiredWithDeterioratedCreditQuality    
Percentage of prospective yield adjustments offset 80.00%onb_PercentageOfProspectiveYieldAdjustmentsOffset      
Loss sharing asset 20,024,000us-gaap_FDICIndemnificationAsset 20,603,000us-gaap_FDICIndemnificationAsset 65,699,000us-gaap_FDICIndemnificationAsset 88,513,000us-gaap_FDICIndemnificationAsset
Loss sharing asset, FDIC indemnification asset 16,600,000onb_LossSharingAssetFdicIndemnificationAsset      
Loss sharing asset, loss share receivable 3,400,000onb_LossSharingAssetLossShareReceivable      
Percentage of increase in indemnification asset measured as a result of impairment 80.00%onb_PercentageOfIncreaseInIndemnificationAssetMeasuredAsResultOfImpairment      
Covered Loans [Member]        
Covered Loans [Line Items]        
Covered loans 136,800,000us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCovered
/ us-gaap_FinancialInstrumentAxis
= onb_CoveredLoansMember		 147,700,000us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCovered
/ us-gaap_FinancialInstrumentAxis
= onb_CoveredLoansMember		    
Outstanding covered loans including principal, interest, fees and penalties 233,400,000us-gaap_FinancingReceivableAcquiredWithDeterioratedCreditQuality
/ us-gaap_FinancialInstrumentAxis
= onb_CoveredLoansMember		 241,900,000us-gaap_FinancingReceivableAcquiredWithDeterioratedCreditQuality
/ us-gaap_FinancialInstrumentAxis
= onb_CoveredLoansMember		    
Expected Indemnification Payments [Member]        
Covered Loans [Line Items]        
Loss sharing asset, FDIC indemnification asset 8,100,000onb_LossSharingAssetFdicIndemnificationAsset
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= onb_ExpectedIndemnificationPaymentsMember		   22,400,000onb_LossSharingAssetFdicIndemnificationAsset
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= onb_ExpectedIndemnificationPaymentsMember		  
To Be Amortized Against Future Accreted Interest Income [Member]        
Covered Loans [Line Items]        
Loss sharing asset, FDIC indemnification asset $ 8,500,000onb_LossSharingAssetFdicIndemnificationAsset
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= onb_ToBeAmortizedAgainstFutureAccretedInterestIncomeMember		   $ 38,200,000onb_LossSharingAssetFdicIndemnificationAsset
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= onb_ToBeAmortizedAgainstFutureAccretedInterestIncomeMember