Fair Value - Quantitative Information about Significant Unobservable Inputs Used in Fair Value Measurements (Detail) $ in Thousands	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment securities - available-for-sale	$ 5,385,091	$ 4,123,416
Significant Unobservable Inputs (Level 3) [Member] | Commercial Real Estate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 11,610	$ 29,125
Valuation Techniques	onb:ValuationTechniqueFairValueOfCollateralMember	onb:ValuationTechniqueFairValueOfCollateralMember
Unobservable Input	us-gaap:MeasurementInputDiscountRateMember	us-gaap:MeasurementInputDiscountRateMember
Loans Held-for-sale, Measurement Input	0.45
Significant Unobservable Inputs (Level 3) [Member] | Commercial Real Estate Foreclosed Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 21
Valuation Techniques	onb:ValuationTechniqueFairValueOfCollateralMember
Unobservable Input	us-gaap:MeasurementInputDiscountRateMember
Loans Held-for-sale, Measurement Input	0.43
Significant Unobservable Inputs (Level 3) [Member] | Commercial Loan [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 10,361	$ 7,242
Valuation Techniques	onb:ValuationTechniqueFairValueOfCollateralMember	onb:ValuationTechniqueFairValueOfCollateralMember
Unobservable Input	us-gaap:MeasurementInputDiscountRateMember	us-gaap:MeasurementInputDiscountRateMember
Significant Unobservable Inputs (Level 3) [Member] | Minimum [Member] | Commercial Real Estate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans Held-for-sale, Measurement Input		0.00
Significant Unobservable Inputs (Level 3) [Member] | Minimum [Member] | Commercial Loan [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans Held-for-sale, Measurement Input	0.00	0.00
Significant Unobservable Inputs (Level 3) [Member] | Maximum [Member] | Commercial Real Estate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans Held-for-sale, Measurement Input		0.50
Significant Unobservable Inputs (Level 3) [Member] | Maximum [Member] | Commercial Loan [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans Held-for-sale, Measurement Input	0.50	0.90
Significant Unobservable Inputs (Level 3) [Member] | Weighted Average [Member] | Commercial Real Estate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans Held-for-sale, Measurement Input		0.35
Significant Unobservable Inputs (Level 3) [Member] | Weighted Average [Member] | Commercial Loan [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans Held-for-sale, Measurement Input	0.13	0.35
Significant Unobservable Inputs (Level 3) [Member] | Pooled Trust Preferred Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment securities - available-for-sale	$ 8,222	$ 8,495
Valuation Techniques	us-gaap:ValuationTechniqueDiscountedCashFlowMember	us-gaap:ValuationTechniqueDiscountedCashFlowMember
Significant Unobservable Inputs (Level 3) [Member] | Pooled Trust Preferred Securities [Member] | Constant Prepayment Rate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.0000	0.0000
Significant Unobservable Inputs (Level 3) [Member] | Pooled Trust Preferred Securities [Member] | Expected Asset Recoveries [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Measurement Input		0.0000
Significant Unobservable Inputs (Level 3) [Member] | Pooled Trust Preferred Securities [Member] | Minimum [Member] | Additional Asset Defaults [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.062	0.068
Significant Unobservable Inputs (Level 3) [Member] | Pooled Trust Preferred Securities [Member] | Minimum [Member] | Expected Asset Recoveries [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.000
Significant Unobservable Inputs (Level 3) [Member] | Pooled Trust Preferred Securities [Member] | Maximum [Member] | Additional Asset Defaults [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.080	0.085
Significant Unobservable Inputs (Level 3) [Member] | Pooled Trust Preferred Securities [Member] | Maximum [Member] | Expected Asset Recoveries [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.191
Significant Unobservable Inputs (Level 3) [Member] | Pooled Trust Preferred Securities [Member] | Weighted Average [Member] | Additional Asset Defaults [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.068	0.073
Significant Unobservable Inputs (Level 3) [Member] | Pooled Trust Preferred Securities [Member] | Weighted Average [Member] | Expected Asset Recoveries [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.060
Significant Unobservable Inputs (Level 3) [Member] | States and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment securities - available-for-sale	$ 40	$ 4,108
Valuation Techniques	us-gaap:ValuationTechniqueDiscountedCashFlowMember	us-gaap:ValuationTechniqueDiscountedCashFlowMember
Significant Unobservable Inputs (Level 3) [Member] | Residential Foreclosed Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 22	$ 68
Valuation Techniques	onb:ValuationTechniqueFairValueOfCollateralMember
Unobservable Input	us-gaap:MeasurementInputDiscountRateMember
Loans Held-for-sale, Measurement Input	0.21
Significant Unobservable Inputs (Level 3) [Member] | Residential Foreclosed Assets [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans Held-for-sale, Measurement Input		0.15
Significant Unobservable Inputs (Level 3) [Member] | Residential Foreclosed Assets [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans Held-for-sale, Measurement Input		0.16
Significant Unobservable Inputs (Level 3) [Member] | Residential Foreclosed Assets [Member] | Weighted Average [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans Held-for-sale, Measurement Input		0.15