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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, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Mar. 08, 2019
Mar. 20, 2018
Earnings Per Share [Abstract]              
Weighted average shares of common stock outstanding (in shares)   8,674,181 7,053,008 8,575,911 7,049,857    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
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   $ 166 $ 196 $ 428 $ 322    
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 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]              
Shares issued under plan (in shares)   35,587   35,587      
Shares fully vested (in shares)   10,744   10,744      
Shares vested during the quarter (in shares)   6,380          
Non-vested number of shares (in shares)   18,463   18,463      
Vesting period (in years)       1 year