Acquisition of New Windsor Bancorp, Inc. - Purchase Price Consideration in Common Stock (Details) - USD ($)	Jul. 01, 2017	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017
Business Acquisition, Equity Interests Issued or Issuable [Line Items]
Shares outstanding		7,034,540	7,023,658		6,067,049
Fair Value of Liabilities Assumed
Goodwill Recorded in Acquisition		$ 19,580,000	$ 19,580,000		$ 6,308,000
NW Bancorp
Business Acquisition, Equity Interests Issued or Issuable [Line Items]
Cash consideration (per New Windsor share) (in dollars per share)	$ 30.00
Cash portion of purchase price	$ 4,519,995
ACNB’s share price for purposes of calculation (in dollars per share)	$ 30.50			$ 30.50
Equity portion of purchase price	$ 28,619,980
Cost of shares owned by buyer	150,000
Total consideration paid	33,289,975
Fair Value of Assets Acquired
Cash and cash equivalents	10,964,000
Investment securities	21,624,000
Loans held for sale	1,463,000
Loans	263,450,000
Restricted stock	486,000
Premises and equipment	8,624,000
Core deposit intangible asset	2,418,000
Other assets	10,792,000
Total assets	319,821,000
Fair Value of Liabilities Assumed
Non-interest bearing deposits	80,006,000
Interest bearing deposits	213,327,000
Subordinated debt	4,688,000
Other liabilities	1,782,000
Total liabilities	299,803,000
Net Assets Acquired	20,018,000
Goodwill Recorded in Acquisition	$ 13,272,000	$ 13,272,000
NW Bancorp | Common Stock
Business Acquisition, Equity Interests Issued or Issuable [Line Items]
Exchange ratio	1.10
Total ACNB shares issued	938,360
NW Bancorp | NW Bancorp
Business Acquisition, Equity Interests Issued or Issuable [Line Items]
Shares outstanding	1,003,703
Shares paid cash consideration	150,555
Shares paid stock consideration	853,148