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Basis of Presentation and Nature of Operations (Details) $ in Thousands |  | 9 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Jul. 02, 2019 director shares | Sep. 30, 2019 USD ($) bank | Mar. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Sep. 30, 2018 USD ($)
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 |  | $ 3,662 |  | $ 0 | $ 0
Lease liability |  | $ 3,662 |  | $ 0 | $ 0
FCBI |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Number of directors joining board of directors | director | 1 |  |  |  |
Pro forma assets |  |  | $ 2,200,000 |  |
Pro forma deposits |  |  | 1,800,000 |  |
Pro forma loans |  |  | $ 1,700,000 |  |
Common Stock | FCBI |  |  |  |  |
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 |  |  |  |

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