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Fair Value - Schedule of Difference between the Aggregate Fair Value and the Aggregate Remaining Principal Balance (Detail) (Residential Loans Held for Sale [Member], USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Mar. 31, 2014
Residential Loans Held for Sale [Member]
    
Fair Value, Estimate Not Practicable, Financial Statement Captions [Line Items]    
Aggregate Fair Value $ 24,344onb_FairValueOfItemsForWhichFairValueOptionWasElected
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= onb_ResidentialLoansHeldForSaleMember		 $ 6,169onb_FairValueOfItemsForWhichFairValueOptionWasElected
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= onb_ResidentialLoansHeldForSaleMember
Difference 592us-gaap_FairValueOptionLoansHeldAsAssetsAggregateDifference
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= onb_ResidentialLoansHeldForSaleMember		 115us-gaap_FairValueOptionLoansHeldAsAssetsAggregateDifference
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= onb_ResidentialLoansHeldForSaleMember
Contractual Principal $ 23,752onb_AggregateUnpaidPrincipalBalanceOfItemsForWhichFairValueOptionWasElected
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= onb_ResidentialLoansHeldForSaleMember		 $ 6,054onb_AggregateUnpaidPrincipalBalanceOfItemsForWhichFairValueOptionWasElected
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= onb_ResidentialLoansHeldForSaleMember