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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details) |  |  |  | 9 Months Ended | 12 Months Ended |  |  |
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Jan. 11, 2020 USD ($) director loan_offices bank $ / shares shares | Feb. 24, 2019 | Jul. 01, 2017 USD ($) | Sep. 30, 2019 | Dec. 31, 2019 USD ($) bank shares | Dec. 31, 2018 USD ($) shares | Dec. 31, 2017 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 |  |  |  |  | 22 |  |  |  |  |
Number of community banking office locations, NWSB | bank |  |  |  |  | 7 |  |  |  |  |
Business Acquisition, Pro Forma Assets |  |  |  |  | $ 1,720,253,000 | $ 1,647,724,000 |  |  |  |
Business Acquisition, Pro Forma Deposits |  |  |  |  | 1,412,260,000 | 1,348,092,000 |  |  |  |
Business Acquisition, Pro Forma Loans |  |  |  |  | 1,258,766,000 | 1,288,501,000 |  |  |  |
Assets Held-in-trust |  |  |  |  | 389,000,000 | 342,000,000 |  |  |  |
Total Loans Receivable |  |  |  |  | $ 1,272,601,000 | 1,302,465,000 |  |  |  |
Number of minimum months for corporation to order updated valuation |  |  |  |  | 18 months |  |  |  |  |
Post-employment benefit cost for continuing life insurance |  |  |  |  | $ 161,000 | 95,000 |  |  |  |
Foreclosed assets held for resale |  |  |  |  | $ 364,000 | $ 155,000 |  |  |  |
2009 Restricted Stock Plan, number of shares authorized, but not issued (in shares) | shares |  |  |  |  |  |  |  | 174,055 |  |
Weighted average number of shares outstanding, basic | shares |  |  |  |  | 7,061,524 | 7,035,818 |  |  |  |
Right of use assets |  |  |  |  | $ 3,502,000 | $ 0 |  |  |  |
Lease liabilities |  |  |  |  | 3,502,000 | 0 |  |  |  |
Restricted Stock |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |
Compensation expense |  |  |  |  | 322,000 | 186,000 |  |  |  |
Credit Concentration Risk |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |
Total Loans Receivable |  |  |  |  | $ 297,654,000 |  |  |  |  |
Concentration Risk, Percentage |  |  |  |  | 23.40% |  |  |  |  |
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 |  |  |  |  | $ 13,438,000 | 14,400,000 |  |  |  |
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 |  |  |  |  | $ 567,048,000 | $ 548,923,000 |  |  |  |
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 |  |  |  |  |
Bank Equipment | Minimum |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |
Useful life |  |  |  |  | 5 years |  |  |  |  |
Bank Equipment | Maximum |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |
Useful life |  |  |  |  | 15 years |  |  |  |  |
New Windsor |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |
Interest rate fair value discount |  |  | $ 731,000 |  |  |  |  |  |  |
Credit fair value discount |  |  | $ 4,500,000 |  |  |  |  |  |  |
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 |  |  |  |  |
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 |  |  |  |  |
Shares fully vested (in shares) | shares |  |  |  |  | 23,730 |  |  |  |  |
Non-vested number of shares (in shares) | shares |  |  |  |  | 2,215 |  |  |  |  |
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 |  |  |  |  | 16,115 |  |  |  |  |
Shares fully vested (in shares) | shares |  |  |  |  | 5,372 |  |  |  |  |
Non-vested number of shares (in shares) | shares |  |  |  |  | 10,743 |  |  |  |  |
Vesting period (in years) |  |  |  | 2 years |  |  |  |  |  |
Subsequent Event |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |
Number of loan production offices | loan_offices | 3 |  |  |  |  |  |  |  |  |
Subsequent Event | FCBI |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |
ACNB’s share price of common stock for purposes of calculation | $ / shares | $ 36.43 |  |  |  |  |  |  |  |  |
Business Combination, Number of Directors Joining Board of Directors | director | 1 |  |  |  |  |  |  |  |  |
Business Acquisition, Pro Forma Assets | $ 2,200,000,000 |  |  |  |  |  |  |  |  |
Business Acquisition, Pro Forma Deposits | 1,800,000,000 |  |  |  |  |  |  |  |  |
Business Acquisition, Pro Forma Loans | $ 1,600,000,000 |  |  |  |  |  |  |  |  |
Business Acquisition, Pro Forma Number of Community Banking Offices | bank | 34 |  |  |  |  |  |  |  |  |
Common Stock | New Windsor |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |
Total ACNB shares of common stock issued | shares |  |  |  |  |  |  | 938,360 |  |  |
Common Stock | Subsequent Event | FCBI |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |
Exchange ratio | shares | 0.9900 |  |  |  |  |  |  |  |  |
Total ACNB shares of common stock issued | shares | 1,590,547 |  |  |  |  |  |  |  |  |
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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