Acquisition of New Windsor Bancorp, Inc. - Narrative (Details)		3 Months Ended
	Jul. 01, 2017 USD ($) bank $ / shares shares	Mar. 31, 2018 USD ($) bank	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Jun. 30, 2017 $ / shares
Business Acquisition [Line Items]
Number of community banking office locations, NWSB | bank		7
Goodwill		$ 19,580,000	$ 6,308,000	$ 19,580,000
Merger related expenses		$ 0	$ 162,000
Core deposit intangibles
Business Acquisition [Line Items]
Amortization period		10 years
New Windsor
Business Acquisition [Line Items]
Number of community banking office locations, NWSB | bank	7
Fair Value of total assets acquired	$ 319,821,000
Fair value of total loans acquired	263,450,000
Fair value of total deposits acquired	293,300,000
Goodwill	13,272,000	$ 13,272,000
Cash portion of purchase price	$ 4,519,995
Percent of post transaction outstanding shares of the Corporation's common stock issued in merger	13.00%
Total consideration paid	$ 33,289,975
ACNB’s share price for purposes of calculation (in dollars per share) | $ / shares	$ 30.50				$ 30.50
Loans receivable, gross amortized cost basis	$ 272,646,000
Interest rate fair value discount	731,000
Credit fair value discount	4,501,000
New Windsor | Long-term Borrowings
Business Acquisition [Line Items]
Liabilities, fair value discount	$ 312,500
New Windsor | Core deposit intangibles
Business Acquisition [Line Items]
Amortization period	10 years	10 years
New Windsor | Time Deposits
Business Acquisition [Line Items]
Assets, fair value premium (discount)	$ 847,500
New Windsor | Common Stock
Business Acquisition [Line Items]
Shares of common stock issued in merger (in shares) | shares	938,360