Cover Page - shares	3 Months Ended
	Mar. 31, 2021	Apr. 30, 2021
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2021
Document Transition Report	false
Entity File Number	1-35015
Entity Registrant Name	ACNB CORP
Entity Incorporation, State or Country Code	PA
Entity Tax Identification Number	23-2233457
Entity Address, Address Line One	16 Lincoln Square
Entity Address, City or Town	Gettysburg
Entity Address, State or Province	PA
Entity Address, Postal Zip Code	17325
City Area Code	717
Local Phone Number	334-3161
Title of 12(b) Security	Common Stock, $2.50 par value per share
Trading Symbol	ACNB
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Document Shell Company Report	false
Entity Common Stock, Shares Outstanding		8,715,020
Entity Central Index Key	0000715579
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q1
Amendment Flag	false