Basis of Presentation and Nature of Operations (Details) $ in Thousands		6 Months Ended
	Jul. 02, 2019 director shares	Jun. 30, 2019 USD ($) bank	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jun. 30, 2018 USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of community banking office locations, ACNB | bank		22
Number of community banking office locations, NWSB | bank		7
Right of use asset		$ 3,711		$ 0	$ 0
Lease liability		$ 3,711		$ 0	$ 0
FCBI
Business Acquisition [Line Items]
Pro forma assets			$ 2,200,000
Pro forma deposits			1,800,000
Pro forma loans			$ 1,700,000
FCBI | Subsequent Event
Business Acquisition [Line Items]
Number of directors joining board of directors | director	1
Common Stock | FCBI | Subsequent Event
Business Acquisition [Line Items]
Approximate number of shares of ACNB common stock expected to be issued in connection with the FCBI acquisition at closing | shares	1,600,596