| Cover Page Document - USD ($) | 12 Months Ended | ||
| Dec. 31, 2019 | Mar. 06, 2020 | Jun. 30, 2019 | |
| Cover page. | |||
| Document Type | 10-K/A | ||
| Amendment Flag | true | ||
| Amendment Description | The Registrant is filing this Amendment No. 1 to Form 10-K for the fiscal year ended December 31, 2019 (“Amendment No. 1”) to correct an error on the cover page of the original Form 10-K as filed with the Securities and Exchange Commission on March 6, 2020 (the “Original Form 10-K”). The cover page of the Original Form 10-K incorrectly showed the total number of shares outstanding. The correct number of shares of the Registrant’s Common Stock outstanding as of March 6, 2020, was 8,669,906, as indicated on the cover page of this Amendment No. 1 (rather than 7,079,359 shares as originally shown, as the result of a typographical error). | ||
| Entity Emerging Growth Company | false | ||
| Entity Small Business | true | ||
| Document Period End Date | Dec. 31, 2019 | ||
| Entity Registrant Name | ACNB CORP | ||
| Entity Central Index Key | 0000715579 | ||
| Current Fiscal Year End Date | --12-31 | ||
| Entity Shell Company | false | ||
| Entity Filer Category | Accelerated Filer | ||
| Entity Common Stock, Shares Outstanding | 8,669,906 | ||
| Document Fiscal Year Focus | 2019 | ||
| Document Fiscal Period Focus | FY | ||
| Entity Interactive Data Current | Yes | ||
| Entity Well-known Seasoned Issuer | No | ||
| Entity Current Reporting Status | Yes | ||
| Entity Public Float | $ 267,627,698 | ||
| Entity Voluntary Filers | No |
