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Acquisition of Frederick County Bancorp, Inc. - Narrative (Details)		3 Months Ended	9 Months Ended
	Jan. 11, 2020 USD ($) bank $ / shares shares	Mar. 31, 2020 USD ($)	Sep. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)
Business Acquisition [Line Items]
Goodwill Recorded in Acquisition			$ 42,108,000	$ 42,108,000	$ 42,108,000
Interest rate fair value discount	$ 2,600,000
Credit fair value discount	5,300,000
Liabilities, Fair Value Adjustment	$ 854,000
Merger related expenses		$ 6,000,000
Frederick County Bancorp, Inc.
Business Acquisition [Line Items]
Number of community banking offices acquired | bank	5
Fair value of total assets acquired	$ 443,400,000
Fair value of total loans acquired	329,312,000
Fair value of total deposits acquired	374,100,000
Goodwill Recorded in Acquisition	$ 22,528,000
ACNB's share price for purposes of calculation (in dollars per share) | $ / shares	$ 36.43
Loans receivable, gross amortized cost basis	$ 339,577,000
Interest rate fair value discount	(2,632,000)
Credit fair value discount	$ (5,931,000)
Frederick County Bancorp, Inc. | Common Stock
Business Acquisition [Line Items]
Exchange ratio | shares	0.9900
Total ACNB shares issued | shares	1,590,547
Investment securities available-for-sale | Frederick County Bancorp, Inc.
Business Acquisition [Line Items]
Assets, fair value premium	$ 163,000
Time deposits | Frederick County Bancorp, Inc.
Business Acquisition [Line Items]
Assets, fair value premium	$ 255,000
Core deposit intangibles
Business Acquisition [Line Items]
Amortization period			10 years
Core deposit intangibles | Frederick County Bancorp, Inc.
Business Acquisition [Line Items]
Amortization period	10 years

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