| Cover | Apr. 29, 2021 | Jan. 28, 2021 |
| Cover [Abstract] | ||
| Document Type | 8-K | |
| Document Period End Date | Apr. 29, 2021 | |
| 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 | false | |
| Soliciting Material | false | |
| Pre-commencement Tender Offer | false | |
| Pre-commencement Issuer Tender Offer | false | |
| Title of 12(b) Security | Common Stock, No Par Value | |
| Trading Symbol | ONB | |
| Security Exchange Name | NASDAQ | |
| Entity Emerging Growth Company | false | |
| Entity Central Index Key | 0000707179 | |
| Amendment Flag | false | |
| Entity Registrant Name | OLD NATIONAL BANCORP |
