Acquisition of Frederick County Bancorp, Inc. - Purchase Price Consideration in Common Stock (Details) - USD ($)	Jan. 11, 2020	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Business Acquisition, Equity Interests Issued or Issuable [Line Items]
Common stock, shares outstanding		8,695,839	7,079,359	7,069,859
Fair Value of Liabilities Assumed
Goodwill Recorded in Acquisition		$ 42,108,000	$ 19,580,000	$ 19,580,000
Common Stock
Business Acquisition, Equity Interests Issued or Issuable [Line Items]
Total ACNB shares issued	1,590,547
Frederick County Bancorp, Inc.
Business Acquisition, Equity Interests Issued or Issuable [Line Items]
Cash consideration (per FCBI share) (in dollars per share)	$ 36.43
Cash portion of purchase price (cash payout of stock options and cash in lieu of fractional shares)	$ 100,798
ACNB's share price for purposes of calculation (in dollars per share)	$ 36.34
Equity portion of purchase price	$ 57,620,595
Cost of shares owned by buyer	187,200
Total consideration paid	57,908,593
Fair Value of Assets Acquired
Cash and cash equivalents	35,262,000
Investment securities	22,167,000
Loans held for sale	4,050,000
Loans	329,312,000
Restricted stock	1,141,000
Premises and equipment	10,959,000
Core deposit intangible asset	3,560,000
Other assets	14,446,000
Total assets	420,897,000
Fair Value of Liabilities Assumed
Non-interest bearing deposits	103,492,000
Interest bearing deposits	270,566,000
Subordinated debt	6,000,000
Long term borrowings	3,450,000
Other liabilities	2,008,000
Total liabilities	385,516,000
Net Assets Acquired	35,381,000
Goodwill Recorded in Acquisition	$ 22,528,000	$ 22,528,000
Frederick County Bancorp, Inc. | Common Stock
Business Acquisition, Equity Interests Issued or Issuable [Line Items]
Exchange ratio	0.9900
Total ACNB shares issued	1,585,597
Frederick County Bancorp, Inc. | Frederick County Bancorp, Inc.
Business Acquisition, Equity Interests Issued or Issuable [Line Items]
Common stock, shares outstanding	1,601,764
Shares paid in cash for fractional shares	150.88
Shares paid stock consideration	1,601,613