Cover Page Document - USD ($)	12 Months Ended
	Dec. 31, 2019	Mar. 06, 2020	Jun. 30, 2019
Cover page.
Document Type	10-K/A
Amendment Flag	true
Amendment Description	The Registrant is filing this Amendment No. 1 to Form 10-K for the fiscal year ended December 31, 2019 (“Amendment No. 1”) to correct an error on the cover page of the original Form 10-K as filed with the Securities and Exchange Commission on March 6, 2020 (the “Original Form 10-K”). The cover page of the Original Form 10-K incorrectly showed the total number of shares outstanding. The correct number of shares of the Registrant’s Common Stock outstanding as of March 6, 2020, was 8,669,906, as indicated on the cover page of this Amendment No. 1 (rather than 7,079,359 shares as originally shown, as the result of a typographical error).
Entity Emerging Growth Company	false
Entity Small Business	true
Document Period End Date	Dec. 31, 2019
Entity Registrant Name	ACNB CORP
Entity Central Index Key	0000715579
Current Fiscal Year End Date	--12-31
Entity Shell Company	false
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		8,669,906
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Entity Interactive Data Current	Yes
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 267,627,698
Entity Voluntary Filers	No