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Earnings Per Share and Restricted Stock (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Feb. 24, 2019
Jun. 30, 2023
Jun. 30, 2022
Jun. 30, 2023
Jun. 30, 2022
Mar. 08, 2019
Mar. 20, 2018
Feb. 24, 2009
Earnings Per Share [Abstract]                
Weighted average shares of common stock outstanding, basic (in shares)   8,512,145 8,705,994 8,517,741 8,695,133      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Weighted average shares of common stock outstanding, basic (in shares)   8,512,145 8,705,994 8,517,741 8,695,133      
Weighted average shares of common stock outstanding, diluted (in shares)   8,523,877 8,705,994 8,541,423 8,695,133      
Non-vested number of shares (in shares)   35,500   35,500        
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   $ 254 $ 116 $ 748 $ 358      
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              
Aggregate shares of common stock under new stock incentive plan (in shares)               200,000
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]                
Non-vested number of shares (in shares)   35,500   35,500        
Shares issued under plan (in shares)   100,746   100,746        
Shares fully vested (in shares)   65,246   65,246        
Shares vested during the quarter (in shares)   0            
Vesting period (in years)       2 years        
Share-Based Compensation Arrangement by Share-Based Payment Award, Number of Shares Available for Grant   473,309   473,309