| Cover | Apr. 20, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Apr. 20, 2021 |
| Entity File Number | 1-35015 |
| Entity Registrant Name | ACNB Corporation |
| Entity Central Index Key | 0000715579 |
| Entity Tax Identification Number | 23-2233457 |
| Entity Incorporation, State or Country Code | PA |
| Entity Address, Address Line One | 16 Lincoln Square |
| Entity Address, City or Town | Gettysburg |
| Entity Address, State or Province | PA |
| Entity Address, Postal Zip Code | 17325 |
| City Area Code | 717 |
| Local Phone Number | 334.3161 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, $2.50 par value per share |
| Trading Symbol | ACNB |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
