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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details) |  |  | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Jan. 11, 2020 USD ($) bank | Feb. 24, 2019 | Dec. 31, 2020 USD ($) bank shares | Dec. 31, 2019 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 |  |  | 21 |  |  |  |
Assets Held-in-trust |  |  | $ 436,700,000 | $ 389,000,000 |  |  |
Total Loans Receivable |  |  | $ 1,637,784,000 | 1,272,601,000 |  |  |
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 |  |  | 139,000 | 161,000 |  |  |
Foreclosed assets held for resale |  |  | $ 0 | $ 364,000 |  |  |
2009 Restricted Stock Plan, number of shares authorized, but not issued (in shares) | shares |  |  |  |  | 174,055 |  |
Weighted average number of shares outstanding, basic | shares |  |  | 8,638,654 | 7,061,524 |  |  |
Right of use assets |  |  | $ 3,145,000 | $ 3,502,000 |  |  |
Lease liabilities |  |  | 3,138,000 | 3,502,000 |  |  |
Restricted Stock |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Compensation expense |  |  | 517,000 | 495,000 |  |  |
Credit Concentration Risk |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Total Loans Receivable |  |  | $ 358,663,000 |  |  |  |
Concentration Risk, Percentage |  |  | 21.90% |  |  |  |
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] |  |  |  |  |  |  |
Total Loans Receivable |  |  | $ 13,151,000 | 13,438,000 |  |  |
Threshold period past due for charge off of financing receivable |  |  | 120 days |  |  |  |
Commercial real estate |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Total Loans Receivable |  |  | $ 744,627,000 | 567,048,000 |  |  |
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% |  |  |  |
Foreclosed assets held for resale |  |  |  | $ 83,000 |  |  |
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 |  |  | 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 |  |  | 7 |  |  |  |
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 |  |  |  |
Accounting Standards Update 2016-02 |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Right of use assets |  |  | $ 4,000,000 |  |  |  |
Lease liabilities |  |  | $ 4,000,000 |  |  |  |
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 |  |  | 17,124 |  |  |  |
Non-vested number of shares (in shares) | shares |  |  | 18,463 |  |  |  |
Vesting period (in years) |  |  | 2 years |  |  |  |
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 |  |  |  |

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