Basis of Presentation and Nature of Operations	3 Months Ended
Basis of Presentation and Nature of Operations	Mar. 31, 2025
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation and Nature of Operations	Basis of Presentation and Nature of Operations ACNB Corporation, headquartered in Gettysburg, Pennsylvania, provides banking, wealth management, mortgage and insurance services to businesses and consumers through its wholly-owned subsidiaries, ACNB Bank and ACNB Insurance Services. The Bank engages in full-service commercial and consumer banking and wealth management services, including trust and retail brokerage, through its 33 community banking offices, including 24 community banking office locations in Adams, Cumberland, Franklin, Lancaster and York Counties, Pennsylvania, and nine community banking office locations in Carroll and Frederick Counties, Maryland. There is also a loan production office in Hunt Valley, Maryland. Recent Acquisition Effective February 1, 2025, ACNB closed the acquisition of Traditions Bancorp, Inc., holding company for Traditions Bank, York, Pennsylvania. Traditions was merged with and into a wholly-owned subsidiary of ACNB Corporation immediately followed by the merger of Traditions Bank with and into ACNB Bank. ACNB Bank is operating the former Traditions Bank offices as “Traditions Bank, A Division of ACNB Bank”. Traditions Bank operated eight community banking offices in South Central Pennsylvania which were included in the Acquisition. In April 2025, ACNB closed two of the eight Traditions Bank branches. ACNB Insurance Services is a full-service insurance agency based in Westminster, Maryland, with additional locations in Jarrettsville, Maryland, and Gettysburg, Pennsylvania. The agency offers a broad range of property, casualty, health, life and disability insurance to both individual and commercial clients. The accompanying unaudited Consolidated Financial Statements have been prepared in accordance with GAAP for interim financial information and with the instructions to Form 10-Q and Article 10 of Regulation S-X. The preparation of financial statements in accordance with GAAP requires management to make estimates and assumptions that affect the amounts of assets and liabilities as of the date of the financial statements as well as revenues and expenses during the period. Actual results could differ from those estimates. In the opinion of management, the accompanying unaudited Consolidated Financial Statements contain all adjustments necessary for a fair presentation. All such adjustments are of a normal recurring nature. These Consolidated Financial Statements should be read in conjunction with the audited Consolidated Financial Statements and the notes included in the Corporation’s Annual Report on Form 10-K for the year ended December 31, 2024. The Corporation evaluates subsequent events through the filing date of this Form 10-Q with the SEC. The results of operations for the three month periods ended March 31, 2025, are not necessarily indicative of the results to be expected for the full year. Certain reclassifications have been made to the prior period financial statements to conform to the current period presentation. Reclassifications had no material effect on prior year net income or stockholders’ equity. Significant Accounting Policies The significant accounting policies used in preparation of the Consolidated Financial Statements are disclosed in the Corporation’s 2024 Annual Report on Form 10-K. Those significant accounting policies are unchanged at March 31, 2025 with the exception of the following modifications or additions: Loans Held for Sale The Corporation previously elected to measure loans originated and intended for sale in the secondary market at the lower of aggregate cost or fair value. Net unrealized losses were recognized through a valuation allowance by charges to income. During the first quarter of 2025, management made the decision to measure loans held for sale at fair value to more appropriately reflect the performance of its entire mortgage banking activities in its Consolidated Financial Statements. The impact of the change in accounting policy for prior periods presented was deemed to be immaterial. Derivative financial instruments related to mortgage banking activities are also recorded at fair value, as detailed under the header “Derivative Financial Instruments and Hedging Activities” below and in Note 9, “Derivative Financial Instruments.” Changes in the fair value of loans held for sale and derivative financial instruments related to mortgage banking during the period are recorded as components of gain from mortgage loans held for sale on the Consolidated Statements of (Loss) Income. Interest income earned on mortgage loans held for sale is classified within interest income on the Consolidated Statements of (Loss) Income. Derivative Instruments and Hedging Activities FASB ASC 815, Derivatives and Hedging, provides the disclosure requirements for derivatives and hedging activities with the intent to provide users of financial statements with an enhanced understanding of: (a) how and why an entity uses derivative instruments, (b) how the entity accounts for derivative instruments and related hedged items, and (c) how derivative instruments and related hedged items affect an entity’s financial position, financial performance, and cash flows. Further, qualitative disclosures are required that explain the Corporation’s objectives and strategies for using derivatives, as well as quantitative disclosures about the fair value of and gains and losses on derivative instruments, and disclosures about credit-risk-related contingent features in derivative instruments. The Corporation records all derivatives on the balance sheet at fair value. The accounting for changes in the fair value of derivatives depends on the intended use of the derivative, whether the Corporation has elected to designate a derivative in a hedging relationship and apply hedge accounting and whether the hedging relationship has satisfied the criteria necessary to apply hedge accounting. Derivatives designated and qualifying as a hedge of the exposure to changes in the fair value of an asset, liability, or firm commitment attributable to a particular risk, such as interest rate risk, are considered fair value hedges. Derivatives designated and qualifying as a hedge of the exposure to variability in expected future cash flows, or other types of forecasted transactions, are considered cash flow hedges. Hedge accounting generally provides for the matching of the timing of gain or loss recognition on the hedging instrument with the recognition of the changes in the fair value of the hedged asset or liability that are attributable to the hedged risk in a fair value hedge or the earnings effect of the hedged forecasted transactions in a cash flow hedge. The Corporation may enter into derivative contracts that are intended to economically hedge certain of its risk, even though hedge accounting does not apply or the Corporation elects not to apply hedge accounting. Acquisition Accounting The Corporation accounts for its mergers and acquisitions using the acquisition method of accounting, in accordance with the provisions of ASC 805 - Business Combinations. Under ASC 805, assets acquired and liabilities assumed in a business combination are at acquisition date fair value and transaction costs and restructuring costs associated with the business combination are expensed as incurred. The determination of fair values involves significant judgment regarding methods and assumptions, including discount rates, future expected cash flows, market conditions and other future events. The excess of the merger consideration over the fair value of assets acquired and liabilities assumed, if any, is allocated to goodwill. The results of operations of the acquired entity are included in the consolidated statements of operations from the Acquisition date. In accordance with business combination accounting guidance, the Company's review of the fair values of the assets and liabilities acquired is ongoing, with management evaluating these fair values for up to one year following the Acquisition date. Adjustments would be recorded to goodwill in the period in which they were determined. Accounting Standards Pending Adoption In December 2023, the FASB issued ASU 2023-09, “Income Taxes (Topic 740)”. This ASU is intended to improve the disclosures for income taxes to address requests from investors, lenders, creditors and other allocators of capital that use the financial statements to make capital allocation decisions. The amendments in ASU 2023-09 will require consistent categories and greater disaggregation of information in the rate reconciliation disclosure as well as disclosure of income taxes paid disaggregated by jurisdiction. The amendments of ASU 2023-09 are effective for annual periods beginning after December 15, 2024, and early adoption is permitted for annual financial statements that have not yet been issued or made available for issuance. The Corporation intends to adopt the amendments of ASU 2023-09 effective January 1, 2025, and will include the required disclosures in its Annual Report on Form 10-K for the year ending December 31, 2025. The Corporation is currently evaluating the impact of this standard, and believes that its adoption will not have a material impact on the Corporation’s Consolidated Financial Statements. In November 2024, the FASB issued ASU 2024-03, “Disaggregation of Income Statement Expenses (Subtopic 220-40)”. This ASU is intended to improve the decision usefulness of expense information on public business entities’ income statements through the disaggregation of relevant expense captions in the notes to the financial statements. The amendments of ASU 2024-03 are effective for annual periods beginning after December 15, 2026, and interim reporting periods beginning after December 15, 2027, and early adoption is permitted. The Corporation is currently evaluating the impact of this standard, and believes that its adoption will not have a material impact on the Corporation’s Consolidated Financial Statements.