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STOCKHOLDERS' EQUITY AND REGULATORY MATTERS (Narrative) (Details) - USD ($) $ in Thousands |  |  | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Jan. 11, 2020 | Feb. 24, 2019 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2017 | Mar. 08, 2019 | Mar. 20, 2018 | Feb. 24, 2009
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |
Shares issued, Dividend Reinvestment and Stock Repurchase Plan (in shares) |  |  | 20,015 | 17,324 |  |  |  |
Proceeds from issuance of common stock, Dividend Reinvestment and Stock Purchase Plan |  |  | $ 493 | $ 501 |  |  |  |
2009 Restricted Stock Plan, number of shares authorized, but not issued (in shares) |  |  |  |  |  | 174,055 |  |
Restricted Stock |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |
Shares issued under plans (in shares) |  |  | 19,472 | 16,115 |  |  |  |
New Windsor |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |
Equity portion of purchase price |  |  |  |  | $ 28,620 |  |  |
FCBI |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |
Equity portion of purchase price | $ 57,721 |  |  |  |  |  |  |
Common Stock |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |
Shares of common stock issued in acquisition (in shares) | 1,590,547 |  |  |  |  |  |  |
Common Stock | New Windsor |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |
Shares of common stock issued in acquisition (in shares) |  |  |  |  | 938,360 |  |  |
Common Stock | FCBI |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |
Shares of common stock issued in acquisition (in shares) | 1,590,547 |  |  |  |  |  |  |
ACNB Corporation 2009 Restricted Stock Plan |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |
2009 Restricted Stock plan, expiration period |  | 10 years |  |  |  |  |  |
Awards authorized (in shares) |  |  |  |  |  |  |  | 200,000
ACNB Corporation 2009 Restricted Stock Plan | Restricted Stock |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |
Shares issued under plan since inception (in shares) |  |  | 25,945 |  |  |  |  |
ACNB Corporation 2018 Omnibus Stock Incentive Plan |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |
Awards authorized (in shares) |  |  |  |  |  |  | 400,000 |
ACNB Corporation 2018 Omnibus Stock Incentive Plan | Restricted Stock |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |
Shares issued under plan since inception (in shares) |  |  | 35,587 |  |  |  |  |

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