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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details)
 12 Months Ended
Feb. 24, 2019
Dec. 31, 2022
USD ($)
bank
shares
Dec. 31, 2021
USD ($)
shares
Mar. 20, 2018
shares
Feb. 24, 2009
shares
Summary of Significant Accounting Policies [Line Items]          
Number of community banking office locations, ACNB | bank   26      
Assets Held-in-trust | $   $ 518,800,000 $ 537,800,000    
Number of minimum months for corporation to order updated valuation   18 months      
Post-employment benefit cost for continuing life insurance | $   $ 81,000 $ 86,000    
Weighted average number of shares outstanding, basic   8,623,012 8,714,926    
Foreclosed assets held for resale | $   $ 474,000 $ 0    
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      
Restricted Stock          
Summary of Significant Accounting Policies [Line Items]          
Compensation expense | $   $ 729,000 $ 110,000    
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      
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%      
Non-Residential Loans | Credit Concentration Risk | Outstanding Debt Benchmark          
Summary of Significant Accounting Policies [Line Items]          
Interest and Fee Income, Loans, Commercial and Residential, Real Estate | $   $ 434,057,000      
Concentration Risk, Percentage   28.20%      
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   5 years      
Furniture and equipment | Maximum          
Summary of Significant Accounting Policies [Line Items]          
Useful life   15 years      
New Windsor          
Summary of Significant Accounting Policies [Line Items]          
Number of community banking office locations acquired | bank   5      
Frederick County Bancorp, Inc.          
Summary of Significant Accounting Policies [Line Items]          
Number of community banking office locations acquired | bank   4      
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        
Awards authorized (in shares)         200,000
ACNB Corporation 2009 Restricted Stock Plan | Restricted Stock          
Summary of Significant Accounting Policies [Line Items]          
Shares issued under plan since inception (in shares)   25,945      
ACNB Corporation 2018 Omnibus Stock Incentive Plan          
Summary of Significant Accounting Policies [Line Items]          
Awards authorized (in shares)       400,000  
ACNB Corporation 2018 Omnibus Stock Incentive Plan | Restricted Stock          
Summary of Significant Accounting Policies [Line Items]          
Shares issued under plan since inception (in shares)   57,522      
Shares fully vested (in shares)   44,154      
Non-vested number of shares (in shares)   13,368      
Vesting period (in years)   1 year      
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