Earnings Per Share and Restricted Stock (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		9 Months Ended
	Feb. 24, 2019	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Mar. 08, 2019	Mar. 20, 2018
Earnings Per Share [Abstract]
Weighted average shares of common stock outstanding, basic (in shares)		8,517,917	8,587,826	8,518,006	8,658,972
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average shares of common stock outstanding, basic (in shares)		8,517,917	8,587,826	8,518,006	8,658,972
Dilutive effect of unvested shares (in shares)		33,628	0	26,726	0
Weighted average shares outstanding, diluted (in shares)		8,551,545	8,587,826	8,544,732	8,658,972
Basic earnings (in dollars per share)		$ 1.06	$ 1.20	$ 3.24	$ 2.95
Diluted earnings (in dollars per share)		$ 1.06	$ 1.20	$ 3.23	$ 2.95
2009 Restricted Stock Plan, Number of shares authorized, but not issued (in shares)						174,055
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense		$ 265	$ 116	$ 1,000	$ 474
ACNB Corporation 2009 Restricted Stock Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
2009 Restricted Stock Plan, expiration period	10 years
ACNB Corporation 2018 Omnibus Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate shares of common stock under new stock incentive plan (in shares)							400,000
ACNB Corporation 2018 Omnibus Stock Incentive Plan | Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Non-vested number of shares (in shares)		33,100		33,100
Shares issued under plan (in shares)		100,746		100,746