STOCKHOLDERS' EQUITY AND REGULATORY MATTERS (Narrative) (Details) $ in Thousands		12 Months Ended
	Feb. 24, 2019	Dec. 31, 2022 USD ($) shares	Dec. 31, 2021 USD ($) shares	Oct. 18, 2022 shares	Mar. 08, 2019 shares	Mar. 20, 2018 shares	Feb. 24, 2009 shares
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Shares issued, Dividend Reinvestment and Stock Repurchase Plan (in shares)		20,908	23,884
Proceeds from issuance of common stock, Dividend Reinvestment and Stock Purchase Plan | $		$ 713	$ 670
2009 Restricted Stock Plan, number of shares authorized, but not issued (in shares)					174,055
Treasury Stock, Shares, Acquired		206,929	54,071
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
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)		57,522
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant		516,533