Basis of Presentation and Nature of Operations	3 Months Ended
Basis of Presentation and Nature of Operations	Mar. 31, 2021
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 its thirty-one community banking offices, including twenty community banking office locations in Adams, Cumberland, Franklin and York Counties, Pennsylvania, as of April 1, 2021. There are also loan production offices situated in Lancaster and York, Pennsylvania, and Hunt Valley, Maryland. RIG is a full-service insurance agency based in Westminster, Maryland, with additional locations in Germantown and Jarrettsville, Maryland. The agency offers a broad range of property and casualty and group life and health insurance to both individual and commercial clients. 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 its marketplace with banking and wealth management services via a network of six community banking offices located in Carroll County, Maryland, as of April 1, 2021. On January 11, 2020, ACNB completed the acquisition of Frederick County Bancorp, Inc. (FCBI), a bank holding company based in Frederick, Maryland. In addition, Frederick County Bank, a Maryland state-chartered bank and FCBI’s wholly-owned subsidiary, merged with and into ACNB Bank. ACNB Bank now operates in the Frederick County, Maryland, market as “FCB Bank, A Division of ACNB Bank” and serves its marketplace with banking and wealth management services via a network of five community banking offices located in Frederick County, Maryland. Further discussion of the FCBI acquisition can be found in Note 2 of these Notes to Consolidated Financial Statements. The Corporation’s primary sources of revenue are 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 (loss) 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 2020 ACNB Corporation Annual Report on Form 10-K, filed with the SEC on March 5, 2021. 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, 2021, are not necessarily indicative of the results to be expected for the full year. ASU 2018-14 The Corporation adopted ASU 2018-14, Compensation—Retirement Benefits—Defined Benefit Plans—General (Topic 715-20): Disclosure Framework—Changes to the Disclosure Requirements for Defined Benefit Plans. The ASU removes the following disclosures: •the amounts in accumulated other comprehensive income that the entity expects to recognize in net periodic benefit cost during the next fiscal year; •the amount and timing of plan assets expected to be returned to the employer; and, •certain related party disclosures. The ASU clarifies the following disclosure requirements: •the projected benefit obligation (PBO) and fair value of plan assets for plans with PBOs in excess of plan assets must be disclosed; and, •the accumulated benefit obligation (ABO) and fair value of plan assets for plans with ABOs in excess of plan assets must be disclosed. The ASU adds the following disclosure requirements: •the weighted-average interest crediting rates for cash balance plans and other plans with promised interest crediting rates; and, •an explanation of the reasons for significant gains and losses related to changes in the benefit obligation for the period. The ASU is effective for public business entities in fiscal years ending after December 15, 2020. Early adoption is permitted. The Corporation is currently evaluating the impact this ASU will have on its consolidated financial condition or results of operations The Corporation has evaluated events and transactions occurring subsequent to the balance sheet date of March 31, 2021, 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.