Basis of Presentation	9 Months Ended
Sep. 30, 2016
Basis Of Accounting [Abstract]
Basis of Presentation	Note 1: Basis of Presentation The accompanying unaudited consolidated financial statements of FSB Bancorp, Inc., (“FSB Bancorp”), FSB Community Bankshares, Inc. (“FSB Community”), Fairport Savings Bank (the “Bank”) and its other wholly owned subsidiary, Fairport Wealth Management (collectively, the “Company”), have been prepared in accordance with accounting principles generally accepted in the United States of America for interim financial information, the instructions for Form 10-Q and Article 8 of Regulation S-X. Accordingly, they do not include all of the information and footnotes necessary for a complete presentation of consolidated financial condition, results of operations and cash flows in conformity with generally accepted accounting principles. In the opinion of management, all adjustments, consisting of normal recurring accruals considered necessary for a fair presentation, have been included. The results are not necessarily indicative of the results that may be expected for the year ending December 31, 2016 or for any future period. References to the Company prior to July 13, 2016 include FSB Community and not FSB Bancorp, whereas after July 13, 2016 references to the Company include FSB Bancorp and not FSB Community. The Company's consolidated financial statements are prepared in accordance with accounting principles generally accepted in the United States and follow practices within the banking industry. Application of these principles requires management to make estimates, assumptions, and judgments that affect the amounts reported in the consolidated financial statements and accompanying notes. These estimates, assumptions, and judgments are based on information available as of the date of the financial statements; accordingly, as this information changes, the financial statements could reflect different estimates, assumptions, and judgments. Certain policies inherently have a greater reliance on the use of estimates, assumptions, and judgments and as such have a greater possibility of producing results that could be materially different than originally reported. Estimates, assumptions, and judgments are necessary when assets and liabilities are required to be recorded at fair value or when an asset or liability needs to be recorded contingent upon a future event. Carrying assets and liabilities at fair value inherently results in more financial statement volatility. The fair values and information used to record valuation adjustments for certain assets and liabilities are based on quoted market prices or are provided by other third-party sources, when available. When third party information is not available, valuation adjustments are estimated in good faith by management. FSB Bancorp is a Maryland Corporation headquartered in Fairport, New York. On July 13, 2016, FSB Bancorp, completed the conversion and reorganization pursuant to which FSB Community Bankshares, MHC converted to the stock holding company form of organization. FSB Bancorp, the new stock holding company for Fairport Savings Bank, sold 1,034,649 shares of common stock, par value $0.01 per share, at $10.00 per share, for gross offering proceeds of $10.3 million in its stock offering. Additionally, after accounting for conversion related expenses of $1.4 million, which offset gross proceeds, the Company received $8.9 million in net proceeds. Concurrent with the completion of the conversion and reorganization, shares of common stock of FSB Community owned by public stockholders were exchanged for shares of FSB Bancorp’s common stock so that the former public stockholders of FSB Community owned approximately the same percentage of FSB Bancorp’s common stock as they owned of FSB Community’s common stock immediately prior to the conversion, subject to adjustment as disclosed in the prospectus. Stockholders of FSB Community received 1.0884 shares of FSB Bancorp’s common stock for each share of FSB Community’s stock they owned immediately prior to completion of the transaction. Cash in lieu of fractional shares was paid based on the offering price of $10.00 per share. All share and per share information in this report for periods prior to the conversion have been revised to reflect the 1.0884:1 conversion ratio on shares outstanding, including shares held by FSB Community Bankshares, MHC that were not publicly traded.

Note 1: Basis of Presentation

The accompanying unaudited consolidated financial statements of FSB Bancorp, Inc., (“FSB Bancorp”), FSB Community Bankshares, Inc. (“FSB Community”), Fairport Savings Bank (the “Bank”) and its other wholly owned subsidiary, Fairport Wealth Management (collectively, the “Company”), have been prepared in accordance with accounting principles generally accepted in the United States of America for interim financial information, the instructions for Form 10-Q and Article 8 of Regulation S-X. Accordingly, they do not include all of the information and footnotes necessary for a complete presentation of consolidated financial condition, results of operations and cash flows in conformity with generally accepted accounting principles. In the opinion of management, all adjustments, consisting of normal recurring accruals considered necessary for a fair presentation, have been included. The results are not necessarily indicative of the results that may be expected for the year ending December 31, 2016 or for any future period. References to the Company prior to July 13, 2016 include FSB Community and not FSB Bancorp, whereas after July 13, 2016 references to the Company include FSB Bancorp and not FSB Community.

The Company's consolidated financial statements are prepared in accordance with accounting principles generally accepted in the United States and follow practices within the banking industry. Application of these principles requires management to make estimates, assumptions, and judgments that affect the amounts reported in the consolidated financial statements and accompanying notes. These estimates, assumptions, and judgments are based on information available as of the date of the financial statements; accordingly, as this information changes, the financial statements could reflect different estimates, assumptions, and judgments. Certain policies inherently have a greater reliance on the use of estimates, assumptions, and judgments and as such have a greater possibility of producing results that could be materially different than originally reported. Estimates, assumptions, and judgments are necessary when assets and liabilities are required to be recorded at fair value or when an asset or liability needs to be recorded contingent upon a future event. Carrying assets and liabilities at fair value inherently results in more financial statement volatility. The fair values and information used to record valuation adjustments for certain assets and liabilities are based on quoted market prices or are provided by other third-party sources, when available. When third party information is not available, valuation adjustments are estimated in good faith by management.

FSB Bancorp is a Maryland Corporation headquartered in Fairport, New York. On July 13, 2016, FSB Bancorp, completed the conversion and reorganization pursuant to which FSB Community Bankshares, MHC converted to the stock holding company form of organization. FSB Bancorp, the new stock holding company for Fairport Savings Bank, sold 1,034,649 shares of common stock, par value $0.01 per share, at $10.00 per share, for gross offering proceeds of $10.3 million in its stock offering. Additionally, after accounting for conversion related expenses of $1.4 million, which offset gross proceeds, the Company received $8.9 million in net proceeds.

Concurrent with the completion of the conversion and reorganization, shares of common stock of FSB Community owned by public stockholders were exchanged for shares of FSB Bancorp’s common stock so that the former public stockholders of FSB Community owned approximately the same percentage of FSB Bancorp’s common stock as they owned of FSB Community’s common stock immediately prior to the conversion, subject to adjustment as disclosed in the prospectus. Stockholders of FSB Community received 1.0884 shares of FSB Bancorp’s common stock for each share of FSB Community’s stock they owned immediately prior to completion of the transaction. Cash in lieu of fractional shares was paid based on the offering price of $10.00 per share. All share and per share information in this report for periods prior to the conversion have been revised to reflect the 1.0884:1 conversion ratio on shares outstanding, including shares held by FSB Community Bankshares, MHC that were not publicly traded.