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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details)
 12 Months Ended
Jan. 11, 2020
USD ($)
bank
Feb. 24, 2019
Dec. 31, 2021
USD ($)
bank
shares
Dec. 31, 2020
USD ($)
shares
Mar. 08, 2019
shares
Mar. 20, 2018
shares
Feb. 24, 2009
shares
Summary of Significant Accounting Policies [Line Items]              
Number of community banking office locations, ACNB | bank     31        
Assets Held-in-trust     $ 537,800,000 $ 436,700,000      
Number of minimum months for corporation to order updated valuation     18 months        
Interest rate fair value discount $ (2,600,000)   $ 2,600,000        
Credit fair value discount $ (5,300,000)   5,900,000        
Post-employment benefit cost for continuing life insurance     $ 86,000 $ 139,000      
2009 Restricted Stock Plan, number of shares authorized, but not issued (in shares) | shares         174,055    
Weighted average number of shares outstanding, basic | shares     8,714,926 8,638,654      
Lease liabilities     $ 3,270,000 $ 3,138,000      
Adams, Cumberland, Franklin and York Counties, Pennsylvania              
Summary of Significant Accounting Policies [Line Items]              
Number of community banking office locations, ACNB | bank     20        
Carroll and Frederick Counties, Maryland              
Summary of Significant Accounting Policies [Line Items]              
Number of community banking office locations, ACNB | bank     11        
Restricted Stock              
Summary of Significant Accounting Policies [Line Items]              
Compensation expense     $ 110,000 $ 517,000      
Credit Concentration Risk              
Summary of Significant Accounting Policies [Line Items]              
Total Loans Receivable     $ 396,795,000        
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]              
Concentration Risk, Percentage     27.00%        
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     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     6        
Frederick County Bancorp, Inc.              
Summary of Significant Accounting Policies [Line Items]              
Number of community banking office locations acquired | bank 5   5        
Interest rate fair value discount $ 2,632,000            
Credit fair value discount $ 5,931,000            
Core Deposits              
Summary of Significant Accounting Policies [Line Items]              
Intangible asset amortization life     10 years        
Core Deposits | Frederick County Bancorp, Inc.              
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) | 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) | shares     25,945        
ACNB Corporation 2018 Omnibus Stock Incentive Plan              
Summary of Significant Accounting Policies [Line Items]              
Awards authorized (in shares) | 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) | shares     35,587        
Shares fully vested (in shares) | shares     29,432        
Non-vested number of shares (in shares) | shares     6,155        
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