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Other Real Estate Owned - Activity in Other Real Estate Owned (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Real Estate Properties [Line Items]      
Beginning Balance     $ 7,241us-gaap_OtherRealEstate
Ending balance 8,482us-gaap_OtherRealEstate 7,629us-gaap_OtherRealEstate 7,241us-gaap_OtherRealEstate
Other Real Estate Owned [Member]      
Real Estate Properties [Line Items]      
Beginning Balance 7,241us-gaap_OtherRealEstate
/ us-gaap_RealEstatePropertiesAxis
= onb_OtherRealEstateOwnedMember		 7,562us-gaap_OtherRealEstate
/ us-gaap_RealEstatePropertiesAxis
= onb_OtherRealEstateOwnedMember		  
Additions 1,906us-gaap_RealEstateOtherAdditions
/ us-gaap_RealEstatePropertiesAxis
= onb_OtherRealEstateOwnedMember		 1,341us-gaap_RealEstateOtherAdditions
/ us-gaap_RealEstatePropertiesAxis
= onb_OtherRealEstateOwnedMember		  
Sales (428)us-gaap_RealEstateCostOfRealEstateSold
/ us-gaap_RealEstatePropertiesAxis
= onb_OtherRealEstateOwnedMember		 (938)us-gaap_RealEstateCostOfRealEstateSold
/ us-gaap_RealEstatePropertiesAxis
= onb_OtherRealEstateOwnedMember		  
Gains (losses)/Write-downs (237)onb_RealEstateOtherChargeOffs
/ us-gaap_RealEstatePropertiesAxis
= onb_OtherRealEstateOwnedMember		 (336)onb_RealEstateOtherChargeOffs
/ us-gaap_RealEstatePropertiesAxis
= onb_OtherRealEstateOwnedMember		  
Ending balance 8,482us-gaap_OtherRealEstate
/ us-gaap_RealEstatePropertiesAxis
= onb_OtherRealEstateOwnedMember		 7,629us-gaap_OtherRealEstate
/ us-gaap_RealEstatePropertiesAxis
= onb_OtherRealEstateOwnedMember		  
Other Real Estate Owned, Covered [Member]      
Real Estate Properties [Line Items]      
Beginning Balance 9,121us-gaap_OtherRealEstate
/ us-gaap_RealEstatePropertiesAxis
= onb_OtherRealEstateOwnedCoveredMember		 13,670us-gaap_OtherRealEstate
/ us-gaap_RealEstatePropertiesAxis
= onb_OtherRealEstateOwnedCoveredMember		  
Additions 360us-gaap_RealEstateOtherAdditions
/ us-gaap_RealEstatePropertiesAxis
= onb_OtherRealEstateOwnedCoveredMember		 4,443us-gaap_RealEstateOtherAdditions
/ us-gaap_RealEstatePropertiesAxis
= onb_OtherRealEstateOwnedCoveredMember		  
Sales (2,556)us-gaap_RealEstateCostOfRealEstateSold
/ us-gaap_RealEstatePropertiesAxis
= onb_OtherRealEstateOwnedCoveredMember		 (4,688)us-gaap_RealEstateCostOfRealEstateSold
/ us-gaap_RealEstatePropertiesAxis
= onb_OtherRealEstateOwnedCoveredMember		  
Gains (losses)/Write-downs 159onb_RealEstateOtherChargeOffs
/ us-gaap_RealEstatePropertiesAxis
= onb_OtherRealEstateOwnedCoveredMember		 (507)onb_RealEstateOtherChargeOffs
/ us-gaap_RealEstatePropertiesAxis
= onb_OtherRealEstateOwnedCoveredMember		  
Ending balance $ 7,084us-gaap_OtherRealEstate
/ us-gaap_RealEstatePropertiesAxis
= onb_OtherRealEstateOwnedCoveredMember		 $ 12,918us-gaap_OtherRealEstate
/ us-gaap_RealEstatePropertiesAxis
= onb_OtherRealEstateOwnedCoveredMember