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Earnings Per Share and Restricted Stock (Details) $ / shares in Units, $ in Thousands |  | 3 Months Ended | 17 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Feb. 24, 2019 | Mar. 31, 2024 USD ($) $ / shares shares | Mar. 31, 2023 USD ($) $ / shares shares | Mar. 31, 2024 shares | Oct. 18, 2022 shares | Mar. 08, 2019 shares | Mar. 20, 2018 shares
Earnings Per Share [Abstract] |  |  |  |  |  |  |
Weighted average shares of common stock outstanding, basic (in shares) |  | 8,493,104 | 8,511,244 |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Weighted average shares of common stock outstanding, basic (in shares) |  | 8,493,104 | 8,511,244 |  |  |  |
Dilutive effect of unvested shares (in shares) |  | 18,544 | 11,732 |  |  |  |
Weighted average shares outstanding, diluted (in shares) |  | 8,511,648 | 8,522,976 |  |  |  |
Basic earnings (in dollars per share) | $ / shares |  | $ 0.80 | $ 1.06 |  |  |  |
Diluted earnings (in dollars per share) | $ / shares |  | $ 0.80 | $ 1.06 |  |  |  |
2009 Restricted Stock Plan, Number of shares authorized, but not issued (in shares) |  |  |  |  |  | 174,055 |
ACNB Corporation 2009 Restricted Stock Plan |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
2009 Restricted Stock Plan, expiration period | 10 years |  |  |  |  |  |
ACNB Corporation 2018 Omnibus Stock Incentive Plan |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Aggregate shares of common stock under new stock incentive plan (in shares) |  |  |  |  |  |  | 400,000
ACNB Corporation 2018 Omnibus Stock Incentive Plan | Restricted Stock |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Non-vested number of shares (in shares) |  | 38,438 |  | 38,438 |  |  |
Shares issued under plan (in shares) |  | 138,019 |  | 138,019 |  |  |
Compensation expense | $ |  | $ 534 | $ 495 |  |  |  |
Stock Repurchase Plan |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Stock Repurchase Program, Number of Shares Authorized to be Repurchased |  |  |  |  | 255,575 |  |
Stock Repurchase Program, Number of Shares Authorized to be Repurchased, Percentage of Outstanding Shares |  |  |  |  | 0.03 |  |
Common stock shares repurchased (in shares) |  | 4,200 | 850 | 65,266 |  |  |

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