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STOCKHOLDERS' EQUITY AND REGULATORY MATTERS (Narrative) (Details) - USD ($)
12 Months Ended
Feb. 24, 2019
Dec. 31, 2019
Dec. 31, 2018
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)   17,324 15,618        
Proceeds from issuance of common stock, Dividend Reinvestment and Stock Purchase Plan   $ 501,000 $ 528,000        
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)   16,115 6,644        
New Windsor              
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]              
Equity portion of purchase price       $ 28,620,000      
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      
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)   16,115