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Loan Servicing Rights - Components of Loan Servicing Rights and Valuation Allowance (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Transfers and Servicing [Abstract]    
Beginning balance $ 9,584us-gaap_ServicingAssetAtAmortizedValue  
Additions 956us-gaap_ServicingAssetAtAmortizedValueAdditions  
Amortization (518)us-gaap_ServicingAssetAtAmortizedValueAmortization1  
Ending balance 10,022us-gaap_ServicingAssetAtAmortizedValue  
Valuation allowance:    
Beginning balance (50)us-gaap_ValuationAllowanceForImpairmentOfRecognizedServicingAssetsBalance  
Additions (437)us-gaap_ValuationAllowanceForImpairmentOfRecognizedServicingAssetsProvisions  
Ending balance (487)us-gaap_ValuationAllowanceForImpairmentOfRecognizedServicingAssetsBalance  
Loan servicing rights, net $ 9,535onb_LoanServicingRightsNet $ 9,500onb_LoanServicingRightsNet