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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