Basis of Presentation and Nature of Operations	3 Months Ended
Mar. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation and Nature of Operations	Basis of Presentation and Nature of Operations ACNB Corporation (the Corporation or ACNB), headquartered in Gettysburg, Pennsylvania, provides banking, insurance, and financial services to businesses and consumers through its wholly-owned subsidiaries, ACNB Bank (Bank) and Russell Insurance Group, Inc. (RIG). The Bank engages in full-service commercial and consumer banking and wealth management services, including trust and retail brokerage, through twenty-two community banking office locations in Adams, Cumberland, Franklin and York Counties, Pennsylvania. There is also a loan production office situated in York County, Pennsylvania. On July 1, 2017, ACNB completed its acquisition of New Windsor Bancorp, Inc. (New Windsor) of Taneytown, Maryland. At the effective time of the acquisition, New Windsor merged with and into a wholly-owned subsidiary of ACNB, immediately followed by the merger of New Windsor State Bank (NWSB) with and into ACNB Bank. ACNB Bank now operates in the Maryland market as “NWSB Bank, A Division of ACNB Bank” and serves this marketplace with banking and wealth management services via a network of seven community banking offices located in Carroll County, Maryland. RIG is a full-service insurance agency based in Westminster, Maryland, with a second location in Germantown, Maryland. The agency offers a broad range of property and casualty, life, and health insurance to both commercial and individual clients. The Corporation’s primary source of revenue is interest income on loans and investment securities and fee income on its products and services. Expenses consist of interest expense on deposits and borrowed funds, provisions for loan losses, and other operating expenses. The accompanying unaudited consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (US GAAP) for interim financial information and with the instructions to Form 10-Q and Article 10 of Regulation S-X. In the opinion of management, the accompanying unaudited consolidated financial statements contain all adjustments necessary to present fairly ACNB Corporation’s financial position and the results of operations, comprehensive income, changes in stockholders’ equity, and cash flows. All such adjustments are of a normal recurring nature. The accounting policies followed by the Corporation are set forth in Note A to the Corporation’s consolidated financial statements in the 2017 ACNB Corporation Annual Report on Form 10-K, filed with the SEC on March 9, 2018. It is suggested that the consolidated financial statements contained herein be read in conjunction with the consolidated financial statements and notes included in the Corporation’s Annual Report on Form 10-K. The results of operations for the three month period ended March 31, 2018, are not necessarily indicative of the results to be expected for the full year. On January 1, 2018, the Corporation adopted ASU 2014-09, Revenue from Contracts with Customers, and all subsequent amendments to the ASU (collectively “ASC 606”), which (i) creates a single framework for recognizing revenue from contracts with customers that fall within its scope and (ii) revises when it is appropriate to recognize a gain (loss) from the transfer of nonfinancial assets, such as OREO. The majority of the Corporation’s revenue comes from interest income, including loans and securities, that are outside the scope of ASC 606. The Corporation’s services that fall within the scope of ASC 606 are presented within other income on the consolidated statement of income and are recognized as revenue as the Corporation satisfies its obligation to the customer. Services within the scope of ASC 606 include service charges on deposit accounts, service charges on ATM and debit card transactions, income from fiduciary investment management and brokerage activities and commission from insurance sales. ASC 606 did not result in a change to the accounting for any in-scope revenue streams; as such, no cumulative effect adjustment was recorded. On January 1, 2018, the Corporation adopted ASU 2016-01, Financial Instruments—Overall (Topic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities, which amended the guidance on the classification and measurement of financial instruments. Upon adoption of ASU 2016-01, the Corporation recognized the equity securities fair value change in net income. Previously the fair value changes were recognized, net of tax, in other comprehensive income (loss). The adoption of this ASU did not have a material effect on the Corporation’s consolidated financial condition or results of operations. The Corporation has evaluated events and transactions occurring subsequent to the balance sheet date of March 31, 2018, for items that should potentially be recognized or disclosed in the consolidated financial statements. The evaluation was conducted through the date these consolidated financial statements were issued.

Basis of Presentation and Nature of Operations

ACNB Corporation (the Corporation or ACNB), headquartered in Gettysburg, Pennsylvania, provides banking, insurance, and financial services to businesses and consumers through its wholly-owned subsidiaries, ACNB Bank (Bank) and Russell Insurance Group, Inc. (RIG). The Bank engages in full-service commercial and consumer banking and wealth management services, including trust and retail brokerage, through twenty-two community banking office locations in Adams, Cumberland, Franklin and York Counties, Pennsylvania. There is also a loan production office situated in York County, Pennsylvania.

On July 1, 2017, ACNB completed its acquisition of New Windsor Bancorp, Inc. (New Windsor) of Taneytown, Maryland. At the effective time of the acquisition, New Windsor merged with and into a wholly-owned subsidiary of ACNB, immediately followed by the merger of New Windsor State Bank (NWSB) with and into ACNB Bank. ACNB Bank now operates in the Maryland market as “NWSB Bank, A Division of ACNB Bank” and serves this marketplace with banking and wealth management services via a network of seven community banking offices located in Carroll County, Maryland.

RIG is a full-service insurance agency based in Westminster, Maryland, with a second location in Germantown, Maryland. The agency offers a broad range of property and casualty, life, and health insurance to both commercial and individual clients.

The Corporation’s primary source of revenue is interest income on loans and investment securities and fee income on its products and services. Expenses consist of interest expense on deposits and borrowed funds, provisions for loan losses, and other operating expenses.

The accompanying unaudited consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (US GAAP) for interim financial information and with the instructions to Form 10-Q and Article 10 of Regulation S-X. In the opinion of management, the accompanying unaudited consolidated financial statements contain all adjustments necessary to present fairly ACNB Corporation’s financial position and the results of operations, comprehensive income, changes in stockholders’ equity, and cash flows. All such adjustments are of a normal recurring nature.

The accounting policies followed by the Corporation are set forth in Note A to the Corporation’s consolidated financial statements in the 2017 ACNB Corporation Annual Report on Form 10-K, filed with the SEC on March 9, 2018. It is suggested that the consolidated financial statements contained herein be read in conjunction with the consolidated financial statements and notes included in the Corporation’s Annual Report on Form 10-K. The results of operations for the three month period ended March 31, 2018, are not necessarily indicative of the results to be expected for the full year.

On January 1, 2018, the Corporation adopted ASU 2014-09, Revenue from Contracts with Customers, and all subsequent amendments to the ASU (collectively “ASC 606”), which (i) creates a single framework for recognizing revenue from contracts with customers that fall within its scope and (ii) revises when it is appropriate to recognize a gain (loss) from the transfer of nonfinancial assets, such as OREO. The majority of the Corporation’s revenue comes from interest income, including loans and securities, that are outside the scope of ASC 606. The Corporation’s services that fall within the scope of ASC 606 are presented within other income on the consolidated statement of income and are recognized as revenue as the Corporation satisfies its obligation to the customer. Services within the scope of ASC 606 include service charges on deposit accounts, service charges on ATM and debit card transactions, income from fiduciary investment management and brokerage activities and commission from insurance sales. ASC 606 did not result in a change to the accounting for any in-scope revenue streams; as such, no cumulative effect adjustment was recorded.

On January 1, 2018, the Corporation adopted ASU 2016-01, Financial Instruments—Overall (Topic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities, which amended the guidance on the classification and measurement of financial instruments. Upon adoption of ASU 2016-01, the Corporation recognized the equity securities fair value change in net income. Previously the fair value changes were recognized, net of tax, in other comprehensive income (loss). The adoption of this ASU did not have a material effect on the Corporation’s consolidated financial condition or results of operations.

The Corporation has evaluated events and transactions occurring subsequent to the balance sheet date of March 31, 2018, for items that should potentially be recognized or disclosed in the consolidated financial statements. The evaluation was conducted through the date these consolidated financial statements were issued.