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ACQUISITION (Narrative) (Details)
 12 Months Ended
Jan. 11, 2020
USD ($)
bank
$ / shares
shares
Dec. 31, 2021
USD ($)
bank
Dec. 31, 2020
USD ($)
Business Acquisition [Line Items]      
Goodwill Recorded in Acquisition   $ 42,108,000 $ 42,108,000
Merger related expenses     $ 6,000,000
Interest rate fair value discount $ 2,600,000 (2,600,000)  
Credit fair value discount 5,300,000 $ (5,900,000)  
Liabilities, Fair Value Adjustment $ 854,000    
Core Deposits      
Business Acquisition [Line Items]      
Amortization period   10 years  
Frederick County Bancorp, Inc.      
Business Acquisition [Line Items]      
Number of community banking office locations acquired | bank 5 5  
Fair value of total assets acquired $ 443,400,000    
Fair value of total loans acquired 329,312,000    
Fair value of total deposits acquired 374,100,000    
Goodwill Recorded in Acquisition $ 22,528,000    
ACNB's share price for purposes of calculation (in dollars per share) | $ / shares $ 36.43    
Loans receivable, gross amortized cost basis $ 339,577,000    
Interest rate fair value discount (2,632,000)    
Credit fair value discount $ (5,931,000)    
Frederick County Bancorp, Inc. | Core Deposits      
Business Acquisition [Line Items]      
Amortization period 10 years    
Frederick County Bancorp, Inc. | Securities (Assets)      
Business Acquisition [Line Items]      
Liabilities, Fair Value Adjustment $ 163,000    
Frederick County Bancorp, Inc. | Bank Time Deposits      
Business Acquisition [Line Items]      
Liabilities, Fair Value Adjustment $ 255,000    
Common Stock | FCBI      
Business Acquisition [Line Items]      
Shares of common stock issued in acquisition (in shares) | shares 1,590,547    
Common Stock | Frederick County Bancorp, Inc.      
Business Acquisition [Line Items]      
Exchange ratio | shares 0.9900    
Shares of common stock issued in acquisition (in shares) | shares 1,590,547