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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details)
$ in Thousands
 12 Months Ended
Feb. 24, 2019
Dec. 31, 2023
USD ($)
bank
shares
Dec. 31, 2022
USD ($)
shares
Jan. 01, 2023
USD ($)
Dec. 31, 2021
USD ($)
Mar. 08, 2019
shares
Mar. 20, 2018
shares
Summary of Significant Accounting Policies [Line Items]              
Number of community banking office locations, ACNB | bank   26          
Assets Held-in-trust   $ 639,400 $ 518,800        
Number of minimum months for corporation to order updated valuation   18 months          
Post-employment benefit cost for continuing life insurance   $ 214 $ 81        
Weighted average shares outstanding, basic (in shares) | shares   8,507,803 8,623,012        
Foreclosed assets held for resale   $ 467 $ 474        
Less: Allowance for credit losses   (19,969) (17,861) $ (17,861) $ (19,033)    
Unfunded Loan Commitment              
Summary of Significant Accounting Policies [Line Items]              
Provision for credit losses   $ (16) 0        
Revision of Prior Period, Adjustment              
Summary of Significant Accounting Policies [Line Items]              
Financing Receivable, Allowance for Credit Loss, Total Adjustment       3,300      
Less: Allowance for credit losses       (1,618)      
Cumulative Effect, Period of Adoption, Adjustment              
Summary of Significant Accounting Policies [Line Items]              
Less: Allowance for credit losses     (1,618)        
Adams, Cumberland, Franklin and York Counties, Pennsylvania              
Summary of Significant Accounting Policies [Line Items]              
Number of community banking office locations, ACNB | bank   17          
Carroll and Frederick Counties, Maryland              
Summary of Significant Accounting Policies [Line Items]              
Number of community banking office locations, ACNB | bank   9          
Minimum              
Summary of Significant Accounting Policies [Line Items]              
Remaining lease terms   1 year          
Initial term greater than   12 months          
Maximum              
Summary of Significant Accounting Policies [Line Items]              
Remaining lease terms   8 years          
Residential Mortgage and Commercial Loans              
Summary of Significant Accounting Policies [Line Items]              
Threshold period past due to discontinue accrual interest on financing receivable   90 days          
Consumer              
Summary of Significant Accounting Policies [Line Items]              
Threshold period past due for charge off of financing receivable   120 days          
Less: Allowance for credit losses   $ (141) (376) (376) (408)    
Consumer | Revision of Prior Period, Adjustment              
Summary of Significant Accounting Policies [Line Items]              
Less: Allowance for credit losses       142      
Consumer | Cumulative Effect, Period of Adoption, Adjustment              
Summary of Significant Accounting Policies [Line Items]              
Less: Allowance for credit losses     142        
Commercial real estate              
Summary of Significant Accounting Policies [Line Items]              
Less: Allowance for credit losses   $ (12,010) (10,016) (10,016) $ (10,716)    
Commercial real estate | Outstanding Debt Benchmark | Credit Concentration Risk              
Summary of Significant Accounting Policies [Line Items]              
Percentage of loan risk   60.90%          
Commercial real estate | Revision of Prior Period, Adjustment              
Summary of Significant Accounting Policies [Line Items]              
Less: Allowance for credit losses       $ (1,106)      
Commercial real estate | Cumulative Effect, Period of Adoption, Adjustment              
Summary of Significant Accounting Policies [Line Items]              
Less: Allowance for credit losses     (1,106)        
Commercial real estate | Maximum              
Summary of Significant Accounting Policies [Line Items]              
Financing receivable term   20 years          
Loan-to-value ratio (no greater than)   80.00%          
Residential Mortgage              
Summary of Significant Accounting Policies [Line Items]              
Loan-to-value ratio that requires private mortgage insurance (in excess of)   80.00%          
Residential Mortgage | Maximum              
Summary of Significant Accounting Policies [Line Items]              
Financing receivable term   30 years          
Loan-to-value ratio (no greater than)   80.00%          
Home Equity Line of Credit | Maximum              
Summary of Significant Accounting Policies [Line Items]              
Financing receivable term   20 years          
Loan-to-value ratio (no greater than)   90.00%          
Buildings              
Summary of Significant Accounting Policies [Line Items]              
Useful life   40 years          
Building Remodels and Additions              
Summary of Significant Accounting Policies [Line Items]              
Useful life   15 years          
Furniture and equipment | Minimum              
Summary of Significant Accounting Policies [Line Items]              
Useful life   3 years          
Furniture and equipment | Maximum              
Summary of Significant Accounting Policies [Line Items]              
Useful life   15 years          
Core Deposits              
Summary of Significant Accounting Policies [Line Items]              
Intangible asset amortization life   10 years          
Customer Lists | Minimum              
Summary of Significant Accounting Policies [Line Items]              
Intangible asset amortization life   8 years          
Customer Lists | Maximum              
Summary of Significant Accounting Policies [Line Items]              
Intangible asset amortization life   15 years          
ACNB Corporation 2009 Restricted Stock Plan              
Summary of Significant Accounting Policies [Line Items]              
2009 Restricted Stock plan, expiration period 10 years            
2009 Restricted Stock Plan, number of shares authorized, but not issued (in shares) | shares           174,055  
ACNB Corporation 2018 Omnibus Stock Incentive Plan | Restricted Stock              
Summary of Significant Accounting Policies [Line Items]              
Shares issued under plan since inception (in shares) | shares   99,381          
Compensation expense   $ 1,000 $ 729        
Non-vested number of shares (in shares) | shares   32,993          
Awards authorized (in shares) | shares             400,000
Real Estate              
Summary of Significant Accounting Policies [Line Items]              
Threshold period past due for Corporation to order updated valuation on financing receivable   90 days          
Number of previous months with no updated valuation completed for Corporation to update validation   12 months          
Number of maximum days of loan being classified as impaired for Corporation to order third party valuation   30 days