ACQUISITION (Narrative) (Details)		12 Months Ended
Jan. 11, 2020 USD ($) director $ / shares shares	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Business Acquisition [Line Items]
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
Merger related expenses		$ 769,000	$ 0
Subsequent Event \| FCBI
Business Acquisition [Line Items]
ACNB’s share price for purposes of calculation (in dollars per share) \| $ / 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
Subsequent Event \| Common Stock \| FCBI
Business Acquisition [Line Items]
Exchange ratio \| shares	0.9900
Shares of common stock issued in acquisition (in shares) \| shares	1,590,547

Business Combination, Equity Interest Issued or Issuable, Number of Shares Per Shareholder

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Business Combination, Number of Directors Joining Board of Directors

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Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
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Number of shares of equity interests issued or issuable to acquire entity.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Price of a single share of a number of saleable stocks paid or offered to be paid in a business combination.

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This element represents acquisition-related costs incurred to effect a business combination which costs have been expensed during the period. Such costs include finder's fees; advisory, legal, accounting, valuation, and other professional or consulting fees; general administrative costs, including the costs of maintaining an internal acquisitions department; and may include costs of registering and issuing debt and equity securities.

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The aggregate of all deposit liabilities held by the entity, including foreign and domestic, interest and noninterest bearing; may include demand deposits, saving deposits, Negotiable Order of Withdrawal (NOW) and time deposits among others.

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Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements and loans held for sale.

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