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STOCKHOLDERS' EQUITY - Narrative (Details) $ in Thousands |  | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Feb. 24, 2019 | Dec. 31, 2023 USD ($) shares | Dec. 31, 2022 USD ($) shares | Oct. 18, 2022 shares | Mar. 08, 2019 shares | Mar. 20, 2018 shares
Stockholders' Equity [Line Items] |  |  |  |  |  |
Treasury stock acquired (in shares) |  |  | 206,929 |  |  |
Dividend Reinvestment Plan |  |  |  |  |  |
Stockholders' Equity [Line Items] |  |  |  |  |  |
Shares issued from the dividend reinvestment plan (in shares) |  | 20,361 | 20,908 |  |  |
Proceeds from the dividend reinvestment plan | $ |  | $ 721 | $ 713 |  |  |
ACNB Corporation 2018 Omnibus Stock Incentive Plan | Restricted Stock |  |  |  |  |  |
Stockholders' Equity [Line Items] |  |  |  |  |  |
Awards authorized (in shares) |  |  |  |  |  | 400,000
Shares issued under plan since inception (in shares) |  | 99,381 |  |  |  |
Non-vested number of shares (in shares) |  | 32,993 |  |  |  |
Compensation expense | $ |  | $ 1,000 | $ 729 |  |  |
ACNB Corporation 2009 Restricted Stock Plan |  |  |  |  |  |
Stockholders' Equity [Line Items] |  |  |  |  |  |
2009 Restricted Stock plan, expiration period | 10 years |  |  |  |  |
2009 Restricted Stock Plan, number of shares authorized, but not issued (in shares) |  |  |  |  | 174,055 |
Stock Repurchase Plan |  |  |  |  |  |
Stockholders' Equity [Line Items] |  |  |  |  |  |
Authorized (in shares) |  |  |  | 255,575 |  |
Percentage of shares authorized |  |  |  | 0.03 |  |
Treasury stock acquired (in shares) |  | 61,066 |  |  |  |

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