| Document and Entity Information | May 30, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | May 30, 2021 |
| Entity Registrant Name | OLD NATIONAL BANCORP |
| Entity Incorporation, State or Country Code | IN |
| Entity File Number | 001-15817 |
| Entity Tax Identification Number | 35-1539838 |
| Entity Address, Address Line One | One Main Street |
| Entity Address, City or Town | Evansville |
| Entity Address, State or Province | IN |
| Entity Address, Postal Zip Code | 47708 |
| City Area Code | 800 |
| Local Phone Number | 731-2265 |
| Written Communications | true |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0000707179 |
| Title of 12(b) Security | Common Stock, No Par Value |
| Trading Symbol | ONB |
| Security Exchange Name | NASDAQ |
