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Fair Value - Quantitative Information about Significant Unobservable Inputs Used in Fair Value Measurements (Parenthetical) (Detail)
 3 Months Ended 12 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Percentage of adjusted specific issuer evaluation defaults 100.00%onb_PercentageOfAdjustedSpecificIssuerEvaluationDefaults 100.00%onb_PercentageOfAdjustedSpecificIssuerEvaluationDefaults
Percentage of adjusted specific issuer evaluation recoveries 100.00%onb_PercentageOfAdjustedSpecificIssuerEvaluationRecoveries 100.00%onb_PercentageOfAdjustedSpecificIssuerEvaluationRecoveries
Minimum [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Percentage of adjusted specific issuer evaluation defaults 0.00%onb_PercentageOfAdjustedSpecificIssuerEvaluationDefaults
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		 0.00%onb_PercentageOfAdjustedSpecificIssuerEvaluationDefaults
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
Minimum [Member] | Pooled Trust Preferred Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Percentage of adjusted specific issuer evaluation recoveries 0.00%onb_PercentageOfAdjustedSpecificIssuerEvaluationRecoveries
/ us-gaap_FairValueByAssetClassAxis
= onb_PooledTrustPreferredSecuritiesMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		 0.00%onb_PercentageOfAdjustedSpecificIssuerEvaluationRecoveries
/ us-gaap_FairValueByAssetClassAxis
= onb_PooledTrustPreferredSecuritiesMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
Maximum [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Percentage of adjusted specific issuer evaluation defaults 50.00%onb_PercentageOfAdjustedSpecificIssuerEvaluationDefaults
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		 50.00%onb_PercentageOfAdjustedSpecificIssuerEvaluationDefaults
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
Maximum [Member] | Pooled Trust Preferred Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Percentage of adjusted specific issuer evaluation recoveries 25.00%onb_PercentageOfAdjustedSpecificIssuerEvaluationRecoveries
/ us-gaap_FairValueByAssetClassAxis
= onb_PooledTrustPreferredSecuritiesMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		 25.00%onb_PercentageOfAdjustedSpecificIssuerEvaluationRecoveries
/ us-gaap_FairValueByAssetClassAxis
= onb_PooledTrustPreferredSecuritiesMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember