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Loans Held for Sale - Summary of Loans Held for Sale (Detail) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Mar. 31, 2014
Loans Held For Sale [Line Items]      
Loans held for sale $ 210,513onb_LoansHeldForSale $ 213,490onb_LoansHeldForSale $ 6,169onb_LoansHeldForSale
Reclassified From Loans held For Investment [Member]      
Loans Held For Sale [Line Items]      
Loans held for sale 186,169onb_LoansHeldForSale
/ invest_InvestmentAxis
= onb_ReclassifiedFromLoansHeldForInvestmentMember		 197,928onb_LoansHeldForSale
/ invest_InvestmentAxis
= onb_ReclassifiedFromLoansHeldForInvestmentMember		  
Commercial Loan [Member] | Reclassified From Loans held For Investment [Member]      
Loans Held For Sale [Line Items]      
Loans held for sale 37,528onb_LoansHeldForSale
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialLoanMember
/ invest_InvestmentAxis
= onb_ReclassifiedFromLoansHeldForInvestmentMember		 45,500onb_LoansHeldForSale
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialLoanMember
/ invest_InvestmentAxis
= onb_ReclassifiedFromLoansHeldForInvestmentMember		  
Commercial Real Estate [Member] | Reclassified From Loans held For Investment [Member]      
Loans Held For Sale [Line Items]      
Loans held for sale 27,081onb_LoansHeldForSale
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialRealEstateMember
/ invest_InvestmentAxis
= onb_ReclassifiedFromLoansHeldForInvestmentMember		 30,690onb_LoansHeldForSale
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialRealEstateMember
/ invest_InvestmentAxis
= onb_ReclassifiedFromLoansHeldForInvestmentMember		  
Residential Real Estate [Member] | Reclassified From Loans held For Investment [Member]      
Loans Held For Sale [Line Items]      
Loans held for sale 68,892onb_LoansHeldForSale
/ us-gaap_FinancialInstrumentAxis
= us-gaap_ResidentialRealEstateMember
/ invest_InvestmentAxis
= onb_ReclassifiedFromLoansHeldForInvestmentMember		 71,680onb_LoansHeldForSale
/ us-gaap_FinancialInstrumentAxis
= us-gaap_ResidentialRealEstateMember
/ invest_InvestmentAxis
= onb_ReclassifiedFromLoansHeldForInvestmentMember		  
Consumer Loan [Member] | Reclassified From Loans held For Investment [Member]      
Loans Held For Sale [Line Items]      
Loans held for sale $ 52,668onb_LoansHeldForSale
/ us-gaap_FinancialInstrumentAxis
= us-gaap_ConsumerLoanMember
/ invest_InvestmentAxis
= onb_ReclassifiedFromLoansHeldForInvestmentMember		 $ 50,058onb_LoansHeldForSale
/ us-gaap_FinancialInstrumentAxis
= us-gaap_ConsumerLoanMember
/ invest_InvestmentAxis
= onb_ReclassifiedFromLoansHeldForInvestmentMember