Accounting Policy (Policies)	3 Months Ended
Mar. 31, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited consolidated financial statements of FSB Bancorp, Inc. (“FSB Bancorp”), 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, 2018 or for any future period. The 2017 FSB Bancorp, Inc. consolidated financial statements, as presented in the Company’s amendment no. 1 to the annual report on Form 10-K/A, should be read in conjunction with these statements. 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.
New Accounting Pronouncements	New Accounting Pronouncements In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) and subsequent updates. This ASU clarifies the principles for recognizing revenue and develops a common standard for U.S. GAAP and International Financial Reporting Standards. The ASU establishes a core principle that requires an entity to identify the contract(s) with a customer, identify the performance obligations in the contract, determine the transaction price, allocate the transaction price to the performance obligations in the contract, and recognize revenue when (or as) the entity satisfies a performance obligation. The ASU provides for improved disclosure requirements that require entities to disclose sufficient financial statements to understand the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers. The Company adopted the guidance effective January 1, 2018 using the modified retrospective method. The Company’s revenue is the sum of net interest income and non-interest income. The scope of the guidance excludes nearly all net interest income as well as many other revenues for financial assets and liabilities including loans, leases, securities, and derivatives. The Company completed its review and determined that the majority of non-interest income revenue streams are within the scope of the new standard. Non-interest income streams that are out of scope of the new standard include BOLI, sales of investment securities, mortgage banking activities, and certain items within service charges and other income. Management reviewed contracts related to service charges on deposits, investment advisory commissions and fee income, insurance commission and fee income, and certain items within other service charges and other income. The Company evaluated the impact of this ASU on the Company’s various revenue streams and, upon adoption on January 1, 2018 and going forward, does not anticipate a material impact to the consolidated financial statements. The Company has included applicable disclosures regarding revenue recognition within Note 9 of these consolidated financial statements. In January 2016, the FASB issued ASU No. 2016-01, Financial Instruments-Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Liabilities. The amendments in this update address certain aspects of recognition, measurement, presentation, and disclosure of financial instruments. The amendments in this update require all equity investments to be measured at fair value with changes in the fair value recognized through net income (other than those accounted for under equity method of accounting or those that result in consolidation of the investee). The amendments in this update also require an entity to present separately in other comprehensive income the portion of the total change in the fair value of a liability resulting from a change in the instrument-specific credit risk when the entity has elected to measure the liability at fair value in accordance with the fair value option for financial instruments. In addition, the amendments in this update also simplify the impairment assessment of equity investments without readily determinable fair values by requiring assessment for impairment qualitatively at each reporting period. For public business entities, the amendments in this update are effective for fiscal years beginning after December 15, 2017, including interim periods with those fiscal years. The adoption had no impact on the consolidated financial statements and only impacted fair value measurement disclosures. 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. The Company adopted the amendment in this update during the three months ended March 31, 2018 and noted no material impact to the consolidated financial statements. In March 2017, the FASB issued an Update (ASU 2017-08) to its guidance on “Receivables – Nonrefundable Fees and Other Costs (Subtopic 310-20) related to premium amortization on purchased callable debt securities. The amendments in this Update shorten the amortization period for certain callable debt securities held at a premium. Specifically, the amendments require the premium to be amortized to the earliest call date. The amendments do not require an accounting change for securities held at a discount; the discount continues to be amortized to maturity. For public business entities, the amendments in this Update are effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2018. Early adoption is permitted, including adoption in an interim period. If an entity early adopts the amendments in an interim period, any adjustments should be reflected as of the beginning of the fiscal year that includes that interim period. An entity should apply the amendments in this Update on a modified retrospective basis through a cumulative-effect adjustment directly to retained earnings as of the beginning of the period of adoption. Additionally, in the period of adoption, an entity should provide disclosure about a change in accounting principle. The adoption of this guidance is not expected to have a material impact on our consolidated results of operations or financial position.

Accounting Policy (Policies)

3 Months Ended

Mar. 31, 2018

The accompanying unaudited consolidated financial statements of FSB Bancorp, Inc. (“FSB Bancorp”), 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, 2018 or for any future period. The 2017 FSB Bancorp, Inc. consolidated financial statements, as presented in the Company’s amendment no. 1 to the annual report on Form 10-K/A, should be read in conjunction with these statements.

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.

New Accounting Pronouncements

New Accounting Pronouncements

In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) and subsequent updates. This ASU clarifies the principles for recognizing revenue and develops a common standard for U.S. GAAP and International Financial Reporting Standards. The ASU establishes a core principle that requires an entity to identify the contract(s) with a customer, identify the performance obligations in the contract, determine the transaction price, allocate the transaction price to the performance obligations in the contract, and recognize revenue when (or as) the entity satisfies a performance obligation. The ASU provides for improved disclosure requirements that require entities to disclose sufficient financial statements to understand the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers. The Company adopted the guidance effective January 1, 2018 using the modified retrospective method. The Company’s revenue is the sum of net interest income and non-interest income. The scope of the guidance excludes nearly all net interest income as well as many other revenues for financial assets and liabilities including loans, leases, securities, and derivatives. The Company completed its review and determined that the majority of non-interest income revenue streams are within the scope of the new standard. Non-interest income streams that are out of scope of the new standard include BOLI, sales of investment securities, mortgage banking activities, and certain items within service charges and other income. Management reviewed contracts related to service charges on deposits, investment advisory commissions and fee income, insurance commission and fee income, and certain items within other service charges and other income. The Company evaluated the impact of this ASU on the Company’s various revenue streams and, upon adoption on January 1, 2018 and going forward, does not anticipate a material impact to the consolidated financial statements. The Company has included applicable disclosures regarding revenue recognition within Note 9 of these consolidated financial statements.

In January 2016, the FASB issued ASU No. 2016-01, Financial Instruments-Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Liabilities. The amendments in this update address certain aspects of recognition, measurement, presentation, and disclosure of financial instruments. The amendments in this update require all equity investments to be measured at fair value with changes in the fair value recognized through net income (other than those accounted for under equity method of accounting or those that result in consolidation of the investee). The amendments in this update also require an entity to present separately in other comprehensive income the portion of the total change in the fair value of a liability resulting from a change in the instrument-specific credit risk when the entity has elected to measure the liability at fair value in accordance with the fair value option for financial instruments. In addition, the amendments in this update also simplify the impairment assessment of equity investments without readily determinable fair values by requiring assessment for impairment qualitatively at each reporting period. For public business entities, the amendments in this update are effective for fiscal years beginning after December 15, 2017, including interim periods with those fiscal years. The adoption had no impact on the consolidated financial statements and only impacted fair value measurement disclosures.

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. The Company adopted the amendment in this update during the three months ended March 31, 2018 and noted no material impact to the consolidated financial statements.

In March 2017, the FASB issued an Update (ASU 2017-08) to its guidance on “Receivables – Nonrefundable Fees and Other Costs (Subtopic 310-20) related to premium amortization on purchased callable debt securities. The amendments in this Update shorten the amortization period for certain callable debt securities held at a premium. Specifically, the amendments require the premium to be amortized to the earliest call date. The amendments do not require an accounting change for securities held at a discount; the discount continues to be amortized to maturity. For public business entities, the amendments in this Update are effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2018. Early adoption is permitted, including adoption in an interim period. If an entity early adopts the amendments in an interim period, any adjustments should be reflected as of the beginning of the fiscal year that includes that interim period. An entity should apply the amendments in this Update on a modified retrospective basis through a cumulative-effect adjustment directly to retained earnings as of the beginning of the period of adoption. Additionally, in the period of adoption, an entity should provide disclosure about a change in accounting principle. The adoption of this guidance is not expected to have a material impact on our consolidated results of operations or financial position.