Nature of Operations and Conversion Plan	6 Months Ended
Jun. 30, 2026
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of Operations and Conversion Plan	NOTE 1. NATURE OF OPERATIONS AND CONVERSION Avidia Bancorp, Inc. (the “Company”) is the bank holding company for Avidia Bank that was created upon the conversion of Assabet Valley Bancorp, the former mutual holding company and sole stockholder of Avidia Bank (the "Bank"), from the mutual form of organization to the stock form of organization. The conversion was completed on July 31, 2025. Prior to July 31, 2025, the conversion had not yet been completed and the Company had no assets or liabilities and had not conducted any business activities other than organizational activities. Accordingly, the unaudited consolidated financial statements, and related notes, and other financial information included in this report at or for any period prior to July 31, 2025 relate to Assabet Valley Bancorp. Conversion and Change in Corporate Form Effective July 31, 2025, Assabet Valley Bancorp, the former mutual holding company of Avidia Bank and the predecessor to Avidia Bancorp, Inc., consummated its mutual to stock conversion and the Company consummated its related stock offering. In the offering, the Company sold 19,176,250 shares of common stock at a per share price of $10.00, including 1,606,100 shares of common stock purchased by the Bank's employee stock ownership plan, for net offering proceeds of approximately $185.8 million. Additionally, the Company donated $1.0 million of cash and 900,000 shares of common stock to the Avidia Bank Charitable Foundation (the "Foundation"). A total of 20,076,250 shares of common stock of the Company were issued and outstanding immediately after the donation to the Foundation. The purchase of the common stock by the ESOP was financed by a loan from the Company. In connection with the conversion, the Company and the Bank established liquidation accounts in an amount equal to Assabet Valley Bancorp’s total equity as reflected in the latest consolidated balance sheets contained in the final offering prospectus for the conversion. The liquidation accounts will be maintained for the benefit of eligible account holders (as defined in the Plan) and supplemental eligible account holders (as defined in the Plan) (collectively, “eligible depositors”) who continue to maintain their deposit accounts in the Bank after the conversion. In the event of a complete liquidation of either (i) the Bank or (ii) the Bank and the Company (and only in such events), eligible depositors who continue to maintain their deposit accounts will be entitled to receive a distribution from the liquidation accounts before any distribution may be made with respect to the common stock of the Company. The Company may not declare or pay a cash dividend if the effect thereof would cause its equity to be reduced below either the amount required for the liquidation accounts or the regulatory capital requirements imposed by its respective bank regulators.

Avidia Bancorp, Inc. (the “Company”) is the bank holding company for Avidia Bank that was created upon the conversion of Assabet Valley Bancorp, the former mutual holding company and sole stockholder of Avidia Bank (the "Bank"), from the mutual form of organization to the stock form of organization. The conversion was completed on July 31, 2025. Prior to July 31, 2025, the conversion had not yet been completed and the Company had no assets or liabilities and had not conducted any business activities other than organizational activities. Accordingly, the unaudited consolidated financial statements, and related notes, and other financial information included in this report at or for any period prior to July 31, 2025 relate to Assabet Valley Bancorp.

Conversion and Change in Corporate Form

Effective July 31, 2025, Assabet Valley Bancorp, the former mutual holding company of Avidia Bank and the predecessor to Avidia Bancorp, Inc., consummated its mutual to stock conversion and the Company consummated its related stock offering. In the offering, the Company sold 19,176,250 shares of common stock at a per share price of $10.00, including 1,606,100 shares of common stock purchased by the Bank's employee stock ownership plan, for net offering proceeds of approximately $185.8 million. Additionally, the Company donated $1.0 million of cash and 900,000 shares of common stock to the Avidia Bank Charitable Foundation (the "Foundation"). A total of 20,076,250 shares of common stock of the Company were issued and outstanding immediately after the donation to the Foundation. The purchase of the common stock by the ESOP was financed by a loan from the Company.

In connection with the conversion, the Company and the Bank established liquidation accounts in an amount equal to Assabet Valley Bancorp’s total equity as reflected in the latest consolidated balance sheets contained in the final offering prospectus for the conversion. The liquidation accounts will be maintained for the benefit of eligible account holders (as defined in the Plan) and supplemental eligible account holders (as defined in the Plan) (collectively, “eligible depositors”) who continue to maintain their deposit accounts in the Bank after the conversion. In the event of a complete liquidation of either (i) the Bank or (ii) the Bank and the Company (and only in such events), eligible depositors who continue to maintain their deposit accounts will be entitled to receive a distribution from the liquidation accounts before any distribution may be made with respect to the common stock of the Company.

The Company may not declare or pay a cash dividend if the effect thereof would cause its equity to be reduced below either the amount required for the liquidation accounts or the regulatory capital requirements imposed by its respective bank regulators.