Loan Servicing Rights - Components of Loan Servicing Rights and Valuation Allowance (Parenthetical) (Detail) $ in Millions	Nov. 30, 2018 USD ($)
KleinBank [Member]
Servicing Assets and Liabilities at Fair Value [Line Items]
Loan servicing rights	$ 0.3

Amount of contract to service financial assets under which the benefits of servicing are expected to more than adequately compensate the servicer, net of valuation allowance for impairment of recognized servicing assets.

No definition available.

Name:	onb_LoanServicingRightsNet
Namespace Prefix:	onb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant