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Loans Held for Sale - Additional Information (Detail) (USD $)
 3 Months Ended
Mar. 31, 2015
Loans
Dec. 31, 2014
Mar. 31, 2014
Servicing Assets at Fair Value [Line Items]      
Loans held for sale $ 210,513,000onb_LoansHeldForSale $ 213,490,000onb_LoansHeldForSale $ 6,169,000onb_LoansHeldForSale
Number of expected transactions for sale of loans 2onb_NumberOfIntendedTransactionsToBeHeldForSaleOfLoans    
Residential Real Estate [Member]      
Servicing Assets at Fair Value [Line Items]      
Mortgage loans held for sale 24,300,000us-gaap_LoansHeldForSaleMortgages
/ us-gaap_FinancialInstrumentAxis
= us-gaap_ResidentialRealEstateMember		 15,600,000us-gaap_LoansHeldForSaleMortgages
/ us-gaap_FinancialInstrumentAxis
= us-gaap_ResidentialRealEstateMember		  
Reclassified From Loans held For Investment [Member]      
Servicing Assets at Fair Value [Line Items]      
Loans held for sale 186,169,000onb_LoansHeldForSale
/ invest_InvestmentAxis
= onb_ReclassifiedFromLoansHeldForInvestmentMember		 197,928,000onb_LoansHeldForSale
/ invest_InvestmentAxis
= onb_ReclassifiedFromLoansHeldForInvestmentMember		  
Reclassified From Loans held For Investment [Member] | Residential Real Estate [Member]      
Servicing Assets at Fair Value [Line Items]      
Loans held for sale $ 68,892,000onb_LoansHeldForSale
/ us-gaap_FinancialInstrumentAxis
= us-gaap_ResidentialRealEstateMember
/ invest_InvestmentAxis
= onb_ReclassifiedFromLoansHeldForInvestmentMember		 $ 71,680,000onb_LoansHeldForSale
/ us-gaap_FinancialInstrumentAxis
= us-gaap_ResidentialRealEstateMember
/ invest_InvestmentAxis
= onb_ReclassifiedFromLoansHeldForInvestmentMember