Accounting Policy (Policies)	6 Months Ended
Jun. 30, 2017
Accounting Policies [Abstract]
Basis of Presentation	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, 2017 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.
New Accounting Pronouncements	New Accounting Pronouncements In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606). This new guidance supersedes the revenue recognition requirements in ASC 605, Revenue Recognition, and is based on the principle that revenue is recognized to depict the transfer of goods or services to customers in an amount that reflects consideration to which the entity expects to be entitled in exchange for those goods and services. The ASU also requires additional disclosure about the nature, amount, timing and uncertainty of revenue and cash flows arising from customer contracts, including significant judgments and changes in judgments and assets recognized from costs incurred to obtain or fulfill a contract. This guidance is effective for the Company for annual and interim periods beginning after December 15, 2017, and should be applied either retrospectively to each prior reporting period presented or retrospectively with the cumulative effect of initially applying this guidance recognized at the date of initial application. The Company's implementation efforts include the identification of revenue within the scope of the guidance, as well as evaluation of revenue contracts. The Company's review is ongoing, and the Company will continue to evaluate any impact as additional guidance is issued and as our internal assessment progresses. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842). This new guidance supersedes the lease requirements in Topic 840, Leases and is based on the principle that a lessee should recognize in the statement of financial position a liability to make lease payments (the lease liability) and a right-of-use asset representing its right to use the underlying asset for the lease term. The accounting applied by a lessor is largely unchanged from that applied under the previous guidance. In addition, the guidance requires an entity to separate the lease components from the nonlease components in a contract. The ASU requires disclosures about the amount, timing, and judgments related to a reporting entity's accounting for leases and related cash flows. The standard is required to be applied to all leases in existence as of the date of adoption using a modified retrospective transition approach. This guidance is effective for fiscal years beginning after December 15, 2018, including interim periods within those fiscal years. Early adoption is permitted for all companies in any interim or annual period. The Company occupies certain offices and uses certain equipment under non-cancelable operating lease agreements, which currently are not reflected in its consolidated statement of condition. The Company expects to recognize lease liabilities and right of use assets associated with these lease agreements; however, the extent of the impact on the Company's consolidated financial statements is currently under evaluation. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments – Credit Losses (Topic 326). This new guidance significantly changes how entities will measure credit losses for most financial assets and certain other instruments that are not measured at fair value through net income. This ASU will replace the "incurred loss" model under existing guidance with an "expected loss" model for instruments measured at amortized cost, and require entities to record allowances for available-for-sale debt securities rather than reduce the carrying amount, as they do today under the other-than-temporary impairment model. This ASU also simplifies the accounting model for purchased credit-impaired debt securities and loans. This guidance requires adoption through a cumulative-effect adjustment to retained earnings as of the beginning of the first reporting period in which the guidance is adopted. This ASU is effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years. Early adoption is permitted for all companies as of fiscal years beginning after December 15, 2018, including interim periods within those fiscal years. The Company is currently evaluating the impact the guidance will have on the Company's consolidated financial statements, and expects an increase in the allowance for credit losses resulting from the change to expected losses for the estimated life of the financial asset, including an allowance for debt securities. The amount of the increase in the allowance for credit losses resulting from the new guidance will be impacted by the portfolio composition and asset quality at the adoption date, as well as economic conditions and forecasts at the time of adoption. In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230). The amendments provide guidance on the following eight specific cash flow issues: 1) debt prepayment or debt extinguishment costs; 2) settlement of zero-coupon debt instruments or other debt instruments with coupon interest rates that are insignificant in relation to the effective interest rate of the borrowing; 3) contingent consideration payments made after a business combination; 4) proceeds from the settlement of insurance claims; 5) proceeds from the settlement of corporate-owned life insurance policies, including bank-owned life insurance policies; 6) distributions received from equity method investees; 7) beneficial interests in securitization transactions; and 8) separately identifiable cash flows and application of the predominance principle. This ASU is effective for fiscal years beginning after December 31, 2017, including interim periods within those fiscal years. Early adoption is permitted, including adoption in an interim period. As this guidance only affects the classification within the statement of cash flows, this ASU is not expected to have a material impact on the Company's consolidated financial statements.

Accounting Policy (Policies)

6 Months Ended

Jun. 30, 2017

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, 2017 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.

New Accounting Pronouncements

New Accounting Pronouncements

In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606). This new guidance supersedes the revenue recognition requirements in ASC 605, Revenue Recognition, and is based on the principle that revenue is recognized to depict the transfer of goods or services to customers in an amount that reflects consideration to which the entity expects to be entitled in exchange for those goods and services. The ASU also requires additional disclosure about the nature, amount, timing and uncertainty of revenue and cash flows arising from customer contracts, including significant judgments and changes in judgments and assets recognized from costs incurred to obtain or fulfill a contract. This guidance is effective for the Company for annual and interim periods beginning after December 15, 2017, and should be applied either retrospectively to each prior reporting period presented or retrospectively with the cumulative effect of initially applying this guidance recognized at the date of initial application. The Company's implementation efforts include the identification of revenue within the scope of the guidance, as well as evaluation of revenue contracts. The Company's review is ongoing, and the Company will continue to evaluate any impact as additional guidance is issued and as our internal assessment progresses.

In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842). This new guidance supersedes the lease requirements in Topic 840, Leases and is based on the principle that a lessee should recognize in the statement of financial position a liability to make lease payments (the lease liability) and a right-of-use asset representing its right to use the underlying asset for the lease term. The accounting applied by a lessor is largely unchanged from that applied under the previous guidance. In addition, the guidance requires an entity to separate the lease components from the nonlease components in a contract. The ASU requires disclosures about the amount, timing, and judgments related to a reporting entity's accounting for leases and related cash flows. The standard is required to be applied to all leases in existence as of the date of adoption using a modified retrospective transition approach. This guidance is effective for fiscal years beginning after December 15, 2018, including interim periods within those fiscal years. Early adoption is permitted for all companies in any interim or annual period. The Company occupies certain offices and uses certain equipment under non-cancelable operating lease agreements, which currently are not reflected in its consolidated statement of condition. The Company expects to recognize lease liabilities and right of use assets associated with these lease agreements; however, the extent of the impact on the Company's consolidated financial statements is currently under evaluation.

In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments – Credit Losses (Topic 326). This new guidance significantly changes how entities will measure credit losses for most financial assets and certain other instruments that are not measured at fair value through net income. This ASU will replace the "incurred loss" model under existing guidance with an "expected loss" model for instruments measured at amortized cost, and require entities to record allowances for available-for-sale debt securities rather than reduce the carrying amount, as they do today under the other-than-temporary impairment model. This ASU also simplifies the accounting model for purchased credit-impaired debt securities and loans. This guidance requires adoption through a cumulative-effect adjustment to retained earnings as of the beginning of the first reporting period in which the guidance is adopted. This ASU is effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years. Early adoption is permitted for all companies as of fiscal years beginning after December 15, 2018, including interim periods within those fiscal years. The Company is currently evaluating the impact the guidance will have on the Company's consolidated financial statements, and expects an increase in the allowance for credit losses resulting from the change to expected losses for the estimated life of the financial asset, including an allowance for debt securities. The amount of the increase in the allowance for credit losses resulting from the new guidance will be impacted by the portfolio composition and asset quality at the adoption date, as well as economic conditions and forecasts at the time of adoption.

In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230). The amendments provide guidance on the following eight specific cash flow issues: 1) debt prepayment or debt extinguishment costs; 2) settlement of zero-coupon debt instruments or other debt instruments with coupon interest rates that are insignificant in relation to the effective interest rate of the borrowing; 3) contingent consideration payments made after a business combination; 4) proceeds from the settlement of insurance claims; 5) proceeds from the settlement of corporate-owned life insurance policies, including bank-owned life insurance policies; 6) distributions received from equity method investees; 7) beneficial interests in securitization transactions; and 8) separately identifiable cash flows and application of the predominance principle. This ASU is effective for fiscal years beginning after December 31, 2017, including interim periods within those fiscal years. Early adoption is permitted, including adoption in an interim period. As this guidance only affects the classification within the statement of cash flows, this ASU is not expected to have a material impact on the Company's consolidated financial statements.