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Earnings Per Share and Restricted Stock (Details) - USD ($) $ in Thousands |  | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | --- | ---
Feb. 24, 2019 | Sep. 30, 2022 | Sep. 30, 2021 | Sep. 30, 2022 | Sep. 30, 2021 | Mar. 08, 2019 | Mar. 20, 2018
Earnings Per Share [Abstract] |  |  |  |  |  |  |
Weighted average shares of common stock outstanding (in shares) |  | 8,587,826 | 8,720,732 | 8,658,972 | 8,715,767 |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Weighted average shares of common stock outstanding (in shares) |  | 8,587,826 | 8,720,732 | 8,658,972 | 8,715,767 |  |
2009 Restricted Stock Plan, Number of shares authorized, but not issued (in shares) |  |  |  |  |  | 174,055 |
Restricted Stock |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Compensation expense |  | $ 116 | $ 28 | $ 474 | $ 83 |  |
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 2009 Restricted Stock Plan | Restricted Stock |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Shares issued under plan (in shares) |  | 25,945 |  | 25,945 |  |  |
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] |  |  |  |  |  |  |
Shares issued under plan (in shares) |  | 57,522 |  | 57,522 |  |  |
Shares fully vested (in shares) |  | 42,899 |  | 42,899 |  |  |
Shares vested during the quarter (in shares) |  | 0 |  |  |  |  |
Non-vested number of shares (in shares) |  | 14,623 |  | 14,623 |  |  |
Vesting period (in years) |  |  |  | 2 years |  |  |

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