Document And Entity Information us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue Over ten years Unfunded Loan Commitments [Member] Related to unfunded loan commitments. us-gaap_DerivativeGainLossOnDerivativeNet (Gain) on fair value hedge Derivative, Gain (Loss) on Derivative, Net Components of Allowance for Credit Losses [Table Text Block] Tabular disclosure for the components of the allowance for credit losses. Note To Financial Statement Details Textual us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasis Available for Sale Amortized cost, Fair value, total fmfg_FinancingReceivableAllowanceForCreditLossIncludingUnfundedCommitments Beginning balance Ending balance The amount of allowance for credit loss, including unfunded commitments, of financing receivables. us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost Within one year Significant Accounting Policies us-gaap_OtherComprehensiveIncomeAvailableforsaleSecuritiesAdjustmentNetOfTaxPortionAttributableToParent Total other comprehensive income us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDate Available for Sale Fair value, total Note 1 - Summary of Significant Accounting Policies us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue Within one year Note 3 - Investment Securities us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue Over one to five years Note 4 - Related Party Transactions us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue Over five to ten years Note 5 - Loans and Allowance for Credit Losses Collateralized Mortgage-Backed Securities [Member] Note 6 - Premises and Equipment Note 7 - Goodwill and Other Intangibles Note 8 - Commitments and Contingencies Note 9 - Interest-bearing Deposits Federal Reserve Bank advances The amount of proceeds from Federal Reserve Bank Advances. Note 10 - Borrowed Funds fmfg_ProvisionForLoanLeaseAndOtherLossesIncludingUnfundedCommitments Provision for (recovery of) credit losses The amount of provision for loan, lease, and other losses, including unfunded commitments. Note 11 - Income Taxes Note 12 - Capital Standards Note 13 - Derivative Financial Instruments fmfg_CertificatesOfDepositInOtherBanksFairValueDisclosure Certificates of deposit in other banks Fair value portion of certificates of deposit in other banks. Note 14 - Fair Value fmfg_AccruedInterestPayableFairValueDisclosure Accrued interest payable Fair value portion of accrued interest payable. Note 15 - Stock-based Compensation us-gaap_OtherComprehensiveIncomeLossAvailableForSaleSecuritiesTax Income tax expense Note 17 - Parent Company Financial Information Note 1 - Summary of Significant Accounting Policies - Basic and Diluted Earnings Per Share (Details) Long-term debt principal payments The cash outflow for debt initially having maturity due after one year or beyond the normal operating cycle, if longer. Note 3 - Investment Securities - Investments in Debt Securities (Details) Amortization of debt issuance costs The net amount of amortization expense attributable to debt issuance costs. Note 3 - Investment Securities - Rating for Held to Maturity Securities (Details) fmfg_AccruedInterestReceivableFairValueDisclosure Accrued interest receivable Fair value portion of accrued interest receivable. Note 3 - Investment Securities - Allowance for Credit Loss on Held to Maturity Securities (Details) US States and Political Subdivisions Debt Securities [Member] Note 3 - Investment Securities - Contractual Maturities (Details) Note 3 - Investment Securities - Gross Unrealized Losses on Continuous Basis for Investment Securities (Details) Note 4 - Related Party Transactions - Related Party Loan Activity (Details) Note 5 - Loans and Allowance for Credit Losses - Major Categories of Loans (Details) Note 5 - Loans and Allowance for Credit Losses - Non-accrual Loans (Details) Note 5 - Loans and Allowance for Credit Losses - Past Due Loans (Details) Note 5 - Loans and Allowance for Credit Losses - Collateral Dependent Loans (Details) us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss 12 months or more Unrealized losses Granted, grant date fair value per share (in dollars per share) Note 5 - Loans and Allowance for Credit Losses - Loan Modifications (Details) us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss Total Unrealized losses Vested, grant date fair value per share (in dollars per share) Note 5 - Loans and Allowance for Credit Losses - Loans by Credit Grade (Details) Forfeited, grant date fair value per share (in dollars per share) us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue Nonvested, grant date fair value per share (in dollars per share) Balance, grant date fair value per share (in dollars per share) Note 5 - Loans and Allowance for Credit Losses - Allowance for Loan Losses (Details) Forfeited, number of shares (in shares) Note 5 - Loans and Allowance for Credit Losses - Amortizable Yield Premium on Purchased Loans (Details) us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber Nonvested, number of shares (in shares) Balance, number of shares (in shares) us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss Less than 12 months Unrealized losses Note 5 - Loans and Allowance for Credit Losses - Components of Allowance for Credit Losses (Details) Derivative [Table] Granted, number of shares (in shares) Note 6 - Premises and Equipment - Premises and Equipment (Details) Corporate Debt Securities [Member] Vested, number of shares (in shares) Note 7 - Goodwill and Other Intangibles - Changes in Goodwill and Other Intangibles (Details) Note 8 - Commitments and Contingencies - Operating Lease Assets and Liabilities (Details) Note 8 - Commitments and Contingencies - Operating Lease Liabilities (Details) Note 8 - Commitment and Contingencies - Outstanding Loan Commitments, Unused Lines of Credit, and Letters of Credit (Details) Other comprehensive income, net of income taxes: Note 9 - Interest-bearing Deposits - Classification of Interest-Bearing Deposits (Details) Note 9 - Interest-bearing Deposits - Certificates of Deposit Maturity Schedule (Details) Note 11 - Borrowed Funds - Borrowed Funds (Details) Note 11 - Income Taxes - Components of Income Tax Expense (Details) Schedule of Derivative Instruments in Statement of Financial Position, Fair Value [Table Text Block] Note 11 - Income Taxes - Reconciliation of Effective Income Tax Rate (Details) Note 11 - Income Taxes - Schedule of Pretax Income (Details) Note 11 - Income Taxes - Schedule of Income Taxes Paid (Details) Rights [Member] Note 11 - Income Taxes - Components of Deferred Tax Expense (Details) Note 12 - Capital Standards - Capital Requirements (Details) us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition Total Fair value Note 13 - Derivative Financial Instruments - Schedule of Derivative Instruments (Details) Note 13 - Derivative Financial Instruments - Derivative Instruments, Gain (Loss) (Details) Note 14 - Fair Value - Summarizes Financial Assets Measured at Fair Value (Details) Note 14 - Fair Value - Assets Measured on a Nonrecurring Basis (Details) us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months Less than 12 months Fair Value us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger 12 months or more Fair value Note 14 - Fair Value - Estimated Fair Value of Financial Instruments (Details) Note 12 - Stock-based Compensation - Summary of Restricted Stock Unit Activity (Details) Note 17 - Parent Company Financial Information - Balance Sheets (Details) Note 17 - Parent Company Financial Information - Balance Sheets (Details) (Parentheticals) Note 17 - Parent Company Financial Information - Statements of Income (Details) Note 17 - Parent Company Financial Information - Statements of Cash Flows (Details) Notes To Financial Statements Notes To Financial Statements [Abstract] Outstanding loan commitments, unused lines of credit, and letters of credit Financial Instruments [Domain] us-gaap_LoansAndLeasesReceivableRelatedPartiesProceeds Amounts collected Financial Instrument [Axis] us-gaap_LoansAndLeasesReceivableRelatedPartiesAdditions Additions us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue Mortgage-backed securities and SBA pools, due in monthly installments us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateAmortizedCost Mortgage-backed securities and SBA pools, due in monthly installments Lessee, Operating Leases [Text Block] Loans, Notes, Trade and Other Receivables Disclosure [Text Block] us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearTwoThroughFiveAmortizedCostBasis Over one to five years us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsGranted Share-Based Compensation Arrangement by Share-Based Payment Award, Non-Option Equity Instruments, Granted Product Liability Contingency [Table] us-gaap_PolicyTextBlockAbstract Accounting Policies us-gaap_DebtSecuritiesAvailableForSaleGainLoss Loss on sale of securities us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod Share-Based Compensation Arrangement by Share-Based Payment Award, Shares Issued in Period Net unrealized gain (loss) on securities available for sale us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage Share-Based Compensation Arrangement by Share-Based Payment Award, Award Vesting Rights, Percentage Share-Based Payment Arrangement, Tranche Three [Member] Liabilities [Abstract] Vesting [Axis] Vesting [Domain] Share-Based Payment Arrangement, Tranche One [Member] Assets us-gaap_Assets Total Assets Share-Based Payment Arrangement, Tranche Two [Member] Plan Name [Axis] Plan Name [Domain] us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized Share-Based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1 Share-Based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition Legal Matters and Contingencies [Text Block] Share-Based Payment Arrangement, Expensed and Capitalized, Amount [Table] Deferred income taxes, net Share-Based Payment Arrangement [Text Block] Other assets Other assets Award Type [Domain] fmfg_PaymentsForProceedsFromLoansMadeToCustomersNetOfPrincipalCollected Loans made to customers, net of principal collected The net amount paid or received by the reporting entity associated with loans made to customers, net of principal collected. Award Type [Axis] us-gaap_FiniteLivedIntangibleAssetsNet Balance, other intangible Balance, other intangible us-gaap_IntangibleAssetsNetExcludingGoodwill Intangible Assets, Net (Excluding Goodwill) Federal funds sold and other interest earning assets Amount of operating interest income from federal funds sold and other interest earning assets. Federal Home Loan Bank of Atlanta advances fmfg_ProceedsFromPaymentsForFederalHomeLoanBankAdvances Amount of cash flow from proceeds of Federal Home Loan Bank (FHLBank) advances. fmfg_PercentageOfOwnership Percentage of Ownership The percentage of ownership owned by the company. First Community Bankers Insurance Co., LLC [Member] First Community Bankers Insurance Co., LLC, a Tennessee “series” limited liability company and licensed property and casualty insurance company. Intercompany balances and transactions have been eliminated. SBA Pools [Member] SBA pools are securities that are made up of pooled loans from the U. S. Small Business Administration. As a Federal agency, the SBA offers loan guarantees through its 7(a) loan program, which allows small businesses that are unable to otherwise secure financing through normal lending channels to obtain loans. Accumulated depreciation and amortization Premises and equipment, net Property, Plant and Equipment, Net us-gaap_Goodwill Balance Balance Property, Plant and Equipment, Gross Government Deposits and Securities Sold Under Repurchase Agreement [Member] Government Deposits and Securities Sold Under Repurchase Agreement. Construction and Land Development [Member] Represents the information pertaining to construction and land development. us-gaap_OtherPostretirementBenefitExpense Other Postretirement Benefits Cost (Reversal of Cost) us-gaap_NotesReceivableGross Financing Receivable, before Allowance for Credit Loss Hedging asset Derivative Assets Securities available for sale Other real estate owned, net fmfg_OtherRealEstateOwnedFairValueDisclosure Other real estate owned, net Fair value portion of other real estate owned. us-gaap_DebtSecurities Debt Securities fmfg_InterestBearingDepositsFairValueDisclosure Interest-bearing deposits Fair value portion of interest-bearing deposits. Derivative Instrument [Axis] fmfg_NoninterestBearingDepositsFairValueDisclosure Noninterest-bearing deposits Fair value portion of noninterest-bearing deposit liabilities. Derivative Contract [Domain] Federal Home Loan Bank Advances [Member] Federal Reserve Bank Advances [Member] Mutual fund Equity security, at fair value Securities Sold under Agreements to Repurchase [Member] Investment in subsidiaries us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures Profit Sharing Plan [Member] Represents information pertaining to profit sharing plan. Letter of Credit [Member] Income before income taxes and equity in undistributed income of subsidiaries us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments Income tax (benefit) Income taxes Total income tax expense Short-Term Debt, Type [Axis] Short-Term Debt, Type [Domain] Gain (Loss) on Securities [Table Text Block] us-gaap_DefinedContributionPlanEmployersMatchingContributionAnnualVestingPercentage Defined Contribution Plan, Employers Matching Contribution, Annual Vesting Percentage us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount Defined Contribution Plan, Employer Discretionary Contribution Amount us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay Federal funds sold and other interest-bearing deposits us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue Within one year us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue Over one to five years us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue Over five to ten years us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue Over ten years Cash and cash equivalents us-gaap_CashAndCashEquivalentsAtCarryingValue us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss Unrealized losses, held to maturity us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain Unrealized gains, held to maturity Salaries Cash and due from banks us-gaap_HeldToMaturitySecuritiesDebtMaturitiesSingleMaturityDateFairValue Held to Maturity Fair value, total Certificates of deposit in other banks Amendment Flag dei_CityAreaCode City Area Code New Accounting Pronouncements, Policy [Policy Text Block] Reclassification, Comparability Adjustment [Policy Text Block] Loss on sale of premises and equipment Gain on sale of SBA loans Gain of sale of SBA loans Current Fiscal Year End Date Loss on sale of debt securities us-gaap_DebtSecuritiesAvailableForSaleRealizedGainLoss Debt Securities, Available-for-Sale, Realized Gain (Loss) Debt Securities, Available-for-Sale, Realized Gain (Loss) Accrued interest receivable Document Fiscal Period Focus Document Fiscal Year Focus Consolidation, Policy [Policy Text Block] dei_DocumentPeriodEndDate Document Period End Date dei_EntityFileNumber Entity File Number Tier One Leverage Capital to Average Assets dei_EntityEmergingGrowthCompany Entity Emerging Growth Company us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue Mortgage-backed securities and SBA pools, due in monthly installments Tier One Leverage Capital Required to be Well Capitalized to Average Assets Tier One Leverage Capital Required to be Well Capitalized to Average Assets dei_DocumentType Document Type Capital Required to be Well Capitalized to Risk Weighted Assets Capital Required to be Well Capitalized to Risk Weighted Assets dei_EntitySmallBusiness Entity Small Business dei_EntityShellCompany Entity Shell Company Common Equity Tier One Capital Ratio Document Information [Line Items] Gain on insurance proceeds, net Gain on insurance proceeds Document Information [Table] Capital to Risk Weighted Assets dei_EntityPublicFloat Entity Public Float dei_EntityFilerCategory Entity Filer Category Tier One Risk Based Capital to Risk Weighted Assets dei_EntityCurrentReportingStatus Entity Current Reporting Status dei_EntityVoluntaryFilers Entity Voluntary Filers Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets dei_EntityWellKnownSeasonedIssuer Entity Well-known Seasoned Issuer Capital Required to be Well Capitalized Award Timing Disclosures [Line Items] Bank owned life insurance Tier One Leverage Capital Tier One Leverage Capital Required to be Well Capitalized Award Timing, How MNPI Considered [Text Block] Common Equity Tier One Capital Statement of Comprehensive Income [Abstract] ecd_AwardTmgMnpiCnsdrdFlag Award Timing MNPI Considered [Flag] dei_EntityTaxIdentificationNumber Entity Tax Identification Number Common Equity Tier One Capital Required to be Well-Capitalized Entity Central Index Key Depreciation and amortization expense Tier One Risk Based Capital Insider Trading Policies and Procedures [Line Items] dei_EntityRegistrantName Entity Registrant Name Tier One Risk Based Capital Required to be Well Capitalized Insider Trading Policies and Procedures Adopted [Flag] Insider Trading Policies and Procedures Adopted [Flag] Entity [Domain] Legal Entity [Axis] dei_EntityAddressAddressLine1 Entity Address, Address Line One us-gaap_FinancingReceivableAllowanceForCreditLosses Financing Receivable, Allowance for Credit Loss us-gaap_AmortizationOfIntangibleAssets Amortization of Intangible Assets Amortization, other intangible ecd_NonRule10b51ArrTrmntdFlag Non-Rule 10b5-1 Arrangement Terminated [Flag] Schedule of Compliance with Regulatory Capital Requirements under Banking Regulations [Table Text Block] dei_EntityAddressCityOrTown Entity Address, City or Town Banking Regulation, Regulatory Capital Requirement, Compliance [Table] us-gaap_DerivativeInstrumentsInHedgesLiabilitiesAtFairValue Fair value hedge ecd_Rule10b51ArrTrmntdFlag Rule 10b5-1 Arrangement Terminated [Flag] ecd_Rule10b51ArrAdoptedFlag Rule 10b5-1 Arrangement Adopted [Flag] dei_EntityAddressPostalZipCode Entity Address, Postal Zip Code Other real estate owned, net Regulatory Capital Requirements under Banking Regulations [Text Block] dei_EntityAddressStateOrProvince Entity Address, State or Province ecd_NonRule10b51ArrAdoptedFlag Non-Rule 10b5-1 Arrangement Adopted [Flag] Material Terms of Trading Arrangement [Text Block] Marketable Securities [Table] dei_EntityCommonStockSharesOutstanding Entity Common Stock, Shares Outstanding (in shares) Trading Arrangements, by Individual [Table] Securities Called at a Premium [Member] Represents securities called at a premium. Investments in Debt and Marketable Equity Securities (and Certain Trading Assets) Disclosure [Text Block] Marketable Securities [Table Text Block] Advertising Advertising Expense Professional services Occupancy Postage, delivery, and armored carrier Amount of expense associated with postage, delivery and armored carrier. us-gaap_HedgedAssetFairValueHedgeCumulativeIncreaseDecrease Securities available for sale dei_LocalPhoneNumber Local Phone Number us-gaap_GainLossOnDispositionOfAssets Loss on disposal of premises and equipment us-gaap_TableTextBlock Notes Tables fmfg_InterestbearingDepositLiabilitiesCertificatesOfDepositPremium Interest-Bearing Deposit Liabilities, Certificates of Deposit, Premium Amount of premium on interest bearing deposit liabilities classified as certificates of deposit. fmfg_AccretionOfPremiumOnCertificateOfDeposits Accretion of Premium on Certificate of Deposits Amount of accretion on premium on certificates of deposits. fmfg_DeferredTaxLiabilitiesInsuranceProceedsFromStormDamage Insurance proceeds from storm damage Amount of deferred tax liability attributable to taxable temporary differences from insurance proceeds from storm damage. us-gaap_FederalHomeLoanBankAdvancesMaturitiesSummaryDueFromOneToTwoYearsOfBalanceSheetDate Federal Home Loan Bank, Advance, Maturity, Year Two us-gaap_FederalHomeLoanBankAdvancesMaturitiesSummaryDueFromTwoToThreeYearsOfBalanceSheetDate Federal Home Loan Bank, Advance, Maturity, Year Three Restricted stock, at cost us-gaap_HeldToMaturitySecuritiesFairValue Fair value, held to maturity Held to Maturity Fair value total us-gaap_CashCashEquivalentsAndFederalFundsSold Cash and cash equivalents Derivative Instruments, Gain (Loss) [Table Text Block] us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankAmountOfAdvancesByBranch Federal Home Loan Bank advances us-gaap_AmortizationOfFinancingCosts Amortization of debt issuance costs us-gaap_DisclosureTextBlockAbstract Notes to Financial Statements Derivative Instruments and Hedging Activities Disclosure [Text Block] Goodwill and other intangibles, net Balance Balance us-gaap_InterestExpenseFederalHomeLoanBankAndFederalReserveBankAdvancesLongTerm Federal Reserve Bank advances Long-term debt us-gaap_InterestExpenseSecuritiesSoldUnderAgreementsToRepurchase Securities sold under repurchase agreements us-gaap_EmployeeBenefitsAndShareBasedCompensation Employee benefits us-gaap_InterestExpenseDeposits Deposits us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo Finite-Lived Intangible Asset, Expected Amortization, Year Two us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearThree Finite-Lived Intangible Asset, Expected Amortization, Year Three us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFour Finite-Lived Intangible Asset, Expected Amortization, Year Four us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFive Finite-Lived Intangible Asset, Expected Amortization, Year Five Segment Reporting, Policy [Policy Text Block] us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths Finite-Lived Intangible Asset, Expected Amortization, Year One Business Combination [Policy Text Block] us-gaap_FiniteLivedIntangibleAssetUsefulLife Finite-Lived Intangible Asset, Useful Life (Year) Earnings Per Share, Policy [Policy Text Block] Interest expense Comprehensive Income, Policy [Policy Text Block] Income Tax, Policy [Policy Text Block] Total income us-gaap_InterestAndDividendIncomeOperating Total interest income us-gaap_FinancingReceivableExcludingAccruedInterestModifiedAccumulated Financing Receivable, Modified us-gaap_LesseeOperatingLeaseTermOfContract Lessee, Operating Lease, Term of Contract (Year) us-gaap_LesseeOperatingLeaseRenewalTerm Lessee, Operating Lease, Renewal Term us-gaap_FinancingReceivableExcludingAccruedInterestRevolvingWriteoff Charge-offs us-gaap_FinancingReceivableExcludingAccruedInterestYearFourOriginatedThreeYearsBeforeCurrentFiscalYearWriteoff Charge-offs Intangible Asset, Indefinite-Lived [Table] us-gaap_FinancingReceivableExcludingAccruedInterestYearFiveOriginatedFourYearsBeforeCurrentFiscalYearWriteoff Charge-offs us-gaap_FinancingReceivableExcludingAccruedInterestOriginatedMoreThanFiveYearsBeforeCurrentFiscalYearWriteoff Charge-offs Cash dividends from subsidiaries us-gaap_InterestAndDividendIncomeSecurities us-gaap_FinancingReceivableModificationToTotalFinancingReceivablesPercent Financing Receivable, Modified us-gaap_FinancingReceivableExcludingAccruedInterestYearThreeOriginatedTwoYearsBeforeCurrentFiscalYearWriteoff Charge-offs us-gaap_FinancingReceivableExcludingAccruedInterestYearOneOriginatedCurrentFiscalYearWriteoff Charge-offs us-gaap_FinancingReceivableModificationToTotalFinancingReceivablesExcludingAccruedInterestPercent Total Financing Receivables, Percentage Share-Based Payment Arrangement [Policy Text Block] us-gaap_FinancingReceivableExcludingAccruedInterestYearTwoOriginatedFiscalYearBeforeCurrentFiscalYearWriteoff Charge-offs us-gaap_FinancingReceivableModifiedWeightedAverageTermIncreaseFromModification Financing Receivable, Modified, Weighted Average Term Increase from Modification us-gaap_TimeDepositMaturitiesYearFour 2029 us-gaap_TimeDepositMaturitiesYearFive 2030 Investment securities - taxable Investment securities - tax exempt us-gaap_TimeDepositMaturitiesYearOne 2026 fmfg_FiniteLivedIntangibleAssetExpectedAmortizationYearSix Finite-Lived Intangible Asset, Expected Amortization, Year Six Amount of amortization for assets, excluding financial assets and goodwill, lacking physical substance with finite life expected to be recognized in the sixth fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach). us-gaap_TimeDepositMaturitiesYearTwo 2027 fmfg_FiniteLivedIntangibleAssetExpectedAmortizationYearSeven Finite-Lived Intangible Asset, Expected Amortization, Year Seven Amount of amortization for assets, excluding financial assets and goodwill, lacking physical substance with finite life expected to be recognized in the seventh fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach). us-gaap_TimeDepositMaturitiesYearThree 2028 Advertising Cost [Policy Text Block] Time Deposit Maturities [Table Text Block] fmfg_IntangibleAssetsNetIncludingGoodwillAmortizationAndImpairment Amortization Amortization amount of intangible assets and impairment amount of goodwill charged during period. us-gaap_InterestAndFeeIncomeLoansAndLeases Loans, including fees fmfg_FinitelivedIntangibleAssetExpectedAmortizationYearEight Finite-Lived Intangible Asset, Expected Amortization, Year Eight Amount of amortization for assets, excluding financial assets and goodwill, lacking physical substance with finite life expected to be recognized in eighth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach). fmfg_FiniteLivedIntangibleAssetExpectedAmortizationYearNine Finite-Lived Intangible Asset, Expected Amortization, Year Nine Amount of amortization for assets, excluding financial assets and goodwill, lacking physical substance with finite life expected to be recognized in ninth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach). fmfg_FinitelivedIntangibleAssetExpectedAmortizationYearTen Finite-Lived Intangible Asset, Expected Amortization, Year Ten Amount of amortization for assets, excluding financial assets and goodwill, lacking physical substance with finite life expected to be recognized in tenth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach). Loan to Fund Merger Consideration [Member] Represents the loan to fund the merger consideration. Revenue from Contract with Customer [Policy Text Block] us-gaap_DeferredTaxAssetsNet Deferred Tax Assets, Net of Valuation Allowance Other time deposits us-gaap_DeferredTaxAssetsLiabilitiesNet Net deferred tax asset us-gaap_DeferredTaxAssetsOther Other Goodwill and Intangible Assets, Policy [Policy Text Block] us-gaap_NoninterestIncome Total noninterest income Other fees and commissions Service charges on deposit accounts Capital Required for Capital Adequacy Amount of minimum total risk-based capital required for capital adequacy as defined by regulatory framework, including capital conversion buffer. Tier One Risk Based Capital Required for Capital Adequacy Amount of minimum Tier 1 risk-based capital required for capital adequacy as defined by regulatory framework, including conversion buffer. us-gaap_DeferredTaxAssetsUnrealizedLossesOnAvailableforSaleSecuritiesGross Unrealized loss on securities available for sale fmfg_PaymentsForEquitySecuritiesDividendReinvested Equity security dividends reinvested Amount of cash outflow for equity securities dividend reinvested. Certificates of deposit, greater than $250,000 Common Equity Tier One Capital Required for Capital Adequacy Ratio Ratio of minimum Common Equity Tier 1 risk-based capital to risk-weighted assets required for capital adequacy as defined by regulatory framework, including conversion buffer. Bank owned life insurance income Tier One Leverage Capital Required for Capital Adequacy to Average Assets Ratio of minimum Tier 1 leverage capital to average assets required for capital adequacy as defined by regulatory framework, including conversion buffer. Deposit Liabilities Disclosures [Text Block] Deposit Liability [Table] Noninterest income Common Equity Tier One Capital Required for Capital Adequacy Amount of minimum Common Equity Tier 1 risk-based capital required for capital adequacy as defined by regulatory framework, including conversion buffer. Tier One Leverage Capital Required for Capital Adequacy Amount of minimum Tier 1 leverage capital required for capital adequacy as defined by regulatory framework, including conversion buffer. Capital Required for Capital Adequacy to Risk Weighted Assets Ratio of minimum total risk-based capital to risk-weighted assets required for capital adequacy as defined by regulatory framework, including conversion buffer. Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets Ratio of minimum Tier 1 risk-based capital to risk-weighted assets required for capital adequacy as defined by regulatory framework, including conversion buffer. us-gaap_InterestIncomeExpenseAfterProvisionForLoanLoss Net interest income after provision for credit losses us-gaap_ProvisionForLoanLossesExpensed Provision for credit losses us-gaap_InterestIncomeExpenseNet Net interest income Property, Plant and Equipment, Estimated Useful Lives (Year) Schedule of Segment Reporting Information, by Segment [Table] us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsProvisionForLoanLosses Allowance for credit losses on loans us-gaap_NumberOfOperatingSegments Number of Operating Segments Furniture and Fixtures [Member] Building and Building Improvements [Member] us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsPostretirementBenefits Post-retirement benefits Land and Land Improvements [Member] us-gaap_InterestAndDebtExpense Interest expense - long-term debt Property, Plant and Equipment, Policy [Policy Text Block] Property, Plant and Equipment [Table] Long-Lived Tangible Asset [Axis] Segment Reporting Disclosure [Text Block] Long-Lived Tangible Asset [Domain] Deposit Account [Member] us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations Cash and cash equivalents at beginning of period Cash and cash equivalents at end of period Assets [Abstract] us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect Net (decrease) increase in cash and cash equivalents Net decrease in cash and cash equivalents us-gaap_NetCashProvidedByUsedInFinancingActivities Net cash provided by financing activities Cash used in financing activities Executive Officer [Member] us-gaap_NetCashProvidedByUsedInOperatingActivities Net cash provided by operating activities Cash provided by operating activities us-gaap_DeferredTaxLiabilitiesOther Other us-gaap_NetCashProvidedByUsedInInvestingActivities Net cash used in investing activities Cash used in investing activities us-gaap_DeferredTaxLiabilities Deferred Tax Liabilities, Net us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLossMaturityAllocatedAndSingleMaturityDate Held to Maturity Amortized cost, total fmfg_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAccretableYieldAmortization Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield, Amortization Amount of amortization recognized for the excess of a loan's cash flows expected to be collected over the investor's initial investment in acquired loans not accounted for as debt securities, with evidence of deterioration of credit quality. Derivatives, Policy [Policy Text Block] us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLossMaturityAllocatedAndSingleMaturityDateRollingAfter5Through10Years Over five to ten years us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLossMaturityAllocatedAndSingleMaturityDateRollingAfterOneThroughFiveYears Over one to five years us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLossMaturityAllocatedAndSingleMaturityDateRollingWithinOneYear Within one year us-gaap_DeferredTaxLiabilitiesLeasingArrangements Right of use asset fmfg_DividendsInExcessOfIncomeOfInsuranceSubsidiary Dividends in excess of income of insurance subsidiary The amount of dividends in excess of income of insurance subsidiary. Equity in undistributed income of bank subsidiary fmfg_EquityInUndistributedIncomeOfBankSubsidiary The amount of equity in undistributed income of bank subsidiary. us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLossMaturityAllocatedAndSingleMaturityDateRollingAfter10Years Over ten years Common Equity Tier One Capital Required to be Well Capitalized Ratio Banking Regulation, Common Equity Tier 1 Risk-Based Capital Ratio, Well Capitalized, Minimum Investment, Policy [Policy Text Block] us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLossMaturityWithoutSingleMaturityDate Mortgage-backed securities and SBA pools, due in monthly installments us-gaap_DeferredTaxLiabilitiesDerivatives Derivatives us-gaap_DeferredTaxLiabilitiesPropertyPlantAndEquipment Depreciation us-gaap_DebtSecuritiesHeldToMaturityExcludingAccruedInterestCreditLossExpenseReversal Provision for credit losses us-gaap_PaymentsOfDividends Dividends paid, net of reinvestments Gain on settlement of fair value hedge Gain on settlement of fair value hedge us-gaap_OtherNoninterestExpense Other Noninterest expense us-gaap_NoninterestExpense Total noninterest expense Furniture and equipment Federal Deposit Insurance Corporation premiums us-gaap_CurrentStateAndLocalTaxExpenseBenefit State us-gaap_DerivativeAssetStatementOfFinancialPositionExtensibleEnumeration Derivative Asset, Statement of Financial Position [Extensible Enumeration] Title and Position [Domain] us-gaap_DeferredStateAndLocalIncomeTaxExpenseBenefit State Title and Position [Axis] Noninterest expense us-gaap_CurrentFederalTaxExpenseBenefit Federal us-gaap_DeferredFederalIncomeTaxExpenseBenefit Federal Collateral Held [Domain] Collateral Pledged [Member] us-gaap_DeferredFederalStateAndLocalTaxExpenseBenefit Total deferred income tax expense us-gaap_DerivativeLiabilityStatementOfFinancialPositionExtensibleEnumeration Derivative Liability, Statement of Financial Position [Extensible Enumeration] us-gaap_IncomeTaxExaminationPenaltiesAndInterestExpense Income Tax Examination, Penalties and Interest Expense Unfunded Loan Commitment [Member] Collateral Held [Axis] us-gaap_CurrentIncomeTaxExpenseBenefit Total current income tax expense Class of Financing Receivable [Domain] Federal Home Loan Bank Stocks [Policy Text Block] Disclosure of accounting policy for federal home loan bank stock. Home Equity Loan [Member] us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesForeign Foreign Unallocated Financing Receivables [Member] Commercial Real Estate Portfolio Segment [Member] Class of Financing Receivable [Axis] Financing Receivable Portfolio Segment [Domain] us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest Income before income taxes Commercial Portfolio Segment [Member] Residential Portfolio Segment [Member] Consumer Portfolio Segment [Member] us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesDomestic Domestic Financing Receivable Portfolio Segment [Axis] Federal Home Loan Bank of Richmond [Member] Federal Home Loan Bank (FHLBank) located in Richmond, Virginia. us-gaap_DebtSecuritiesAvailableForSaleAccruedInterestAfterAllowanceForCreditLoss Debt Securities, Available-for-Sale, Accrued Interest, after Allowance for Credit Loss dei_IcfrAuditorAttestationFlag ICFR Auditor Attestation Flag us-gaap_DebtSecuritiesAvailableForSaleAccruedInterestAfterAllowanceForCreditLossStatementOfFinancialPositionExtensibleList Debt Securities, Available-for-Sale, Accrued Interest, after Allowance for Credit Loss, Statement of Financial Position [Extensible Enumeration] Mortgage Banking Activity [Policy Text Block] us-gaap_PaymentsOfDebtIssuanceCosts Payments of Debt Issuance Costs Financing Receivable, Real Estate Acquired Through Foreclosure [Policy Text Block] Loans, 90 days past due and still accruing Nonaccrual, No Allowance Still accruing Schedule of Credit Commitments [Table Text Block] The tabular disclosure for outstanding credit commitments. Income Tax Jurisdiction [Axis] Income Tax Jurisdiction [Domain] The 2023 Equity Compensation Plan [Member] Represents the 2023 Equity Compensation Plan. us-gaap_DebtSecuritiesHeldToMaturityAccruedInterestAfterAllowanceForCreditLossStatementOfFinancialPositionExtensibleList Debt Securities, Held-to-Maturity, Accrued Interest, after Allowance for Credit Loss, Statement of Financial Position [Extensible Enumeration] Unsecured Line of Credit [Member] An unsecured contractual arrangement with a lender under which borrowings can be made up to a specific amount at any point in time, and under which borrowings outstanding may be either short-term or long-term, depending upon the particulars. Domestic Tax Jurisdiction [Member] us-gaap_DebtSecuritiesHeldToMaturityAccruedInterestAfterAllowanceForCreditLoss Debt Securities, Held-to-Maturity, Accrued Interest, after Allowance for Credit Loss Commercial Lines [Member] Represents the unused commercial lines of credit. us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossRecovery Recoveries us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossWriteoff Charge-offs Charge-offs Charge-offs Write down of other real estate owned Represents the amount of write down on other real estate owned. Cash and Cash Equivalents, Policy [Policy Text Block] us-gaap_FinancingReceivableExcludingAccruedInterestCreditLossExpenseReversal Provision for loan losses us-gaap_FinancingReceivableExcludingAccruedInterestNonaccrual Financing Receivable, Excluding Accrued Interest, Nonaccrual Construction and land development Nonaccrual, No Allowance Accounting Policies [Abstract] Significant Accounting Policies [Text Block] Nonaccrual loans Nonaccrual loans, with allowance Amortized cost of financing receivable on nonaccrual status with allowance for credit loss. Excludes net investment in lease. us-gaap_CommonStockDividendsPerShareCashPaid Common Stock, Dividends, Per Share, Cash Paid (in dollars per share) fmfg_DeferredTaxAssetsNonaccrualInterest Nonaccrual interest Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from nonaccrual interest. fmfg_DeferredTaxAssetsOtherRealEstateOwnedAllowanceForLoss Other real estate owned valuation adjustments Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from other real estate owned allowance for loss.. us-gaap_ProceedsFromIssuanceOfLongTermDebt Proceeds from Issuance of Long-Term Debt Subordinated debt offering, net of issuance costs Subordinated debt offering, net of issuance costs Condensed Financial Information of Parent Company Only Disclosure [Text Block] Impaired Loans [Member] Represents impaired loans. Other Real Estate Owned [Member] Represents the fair value of other real estate owned. Tax Period [Domain] Schedule of Accounts, Notes, Loans and Financing Receivable [Table Text Block] Accounts and Financing Receivables [Table] Income before equity in undistributed income of subsidiaries fmfg_IncomeLossFromContinuingOperationsBeforeEquityMethodInvestmentsNoncontrollingInterest Amount of income (loss) from continuing operations before deduction of income (loss) attributable to noncontrolling interest, and addition of income (loss) from equity method investments. dei_AuditorName Auditor Name Tax Period [Axis] Auditor Firm ID dei_AuditorLocation Auditor Location Automated teller machine and debit card expenses Amount of expense recognized during the period for automated teller machines and debt card expense. us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment Weighted Average Number of Shares Outstanding, Diluted, Adjustment Pledging Purpose [Domain] us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding Weighted average shares outstanding (in shares) Bank Owned Life Insurance, Policy [Policy Text Block] Disclosure of accounting policy for life insurance. Equity Securities, Policy [Policy Text Block] Disclosure of accounting policy for equity securities. Pledging Purpose [Axis] us-gaap_FinancingReceivableAccruedInterestAfterAllowanceForCreditLossStatementOfFinancialPositionExtensibleList Financing Receivable, Accrued Interest, after Allowance for Credit Loss, Statement of Financial Position [Extensible Enumeration] us-gaap_FinancingReceivableAccruedInterestAfterAllowanceForCreditLoss Financing Receivable, Accrued Interest, after Allowance for Credit Loss Asset Class [Axis] Asset Class [Domain] Statement of Financial Position [Abstract] Earnings per common share - diluted (in dollars per share) Weighted average shares outstanding - basic (in shares) us-gaap_EffectiveIncomeTaxRateContinuingOperations Actual income tax expense Earnings per common share - basic (in dollars per share) Credit Loss, Financial Instrument [Policy Text Block] Statement of Cash Flows [Abstract] Statement of Stockholders' Equity [Abstract] us-gaap_ProceedsFromPaymentsForInSecuritiesSoldUnderAgreementsToRepurchase Securities sold under repurchase agreements fmfg_LesseeOperatingLeaseAdditionalRenewalTermNumber Lessee, Operating Lease, Additional Renewal Term Number Number of additional renewal terms under operating lease option. Operating Lease for Hampstead Branch [Member] Operating lease for the land on which the Hampstead branch. Financing Receivable, Nonaccrual [Table Text Block] fmfg_LesseeOperatingLeaseNumberOfRenewalTerms Lessee, Operating Lease, Number of Renewal Terms Represents number of renewal terms for operating lease. Mortgage banking income Operating Lease for Certain Facilities of Greenmount Branch [Member] Operating lease for certain facilities where the Greenmount branch is located. Operating Lease for the Corporate Headquarters [Member] Operating lease for the corporate headquarters. Schedule of Income before Income Tax, Domestic and Foreign [Table Text Block] Lease for Carroll Lutheran Village Branch [Member] Operating lease for its Carroll Lutheran Village branch. Investments Classified by Contractual Maturity Date [Table Text Block] us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseLifeInsurance Appreciation of cash surrender value of life insurance Schedule of Components of Income Tax Expense (Benefit) [Table Text Block] us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseOther Other Schedule of Deferred Tax Assets and Liabilities [Table Text Block] Schedule of Effective Income Tax Rate Reconciliation [Table Text Block] Debt Securities, Held-to-Maturity, Allowance for Credit Loss [Table Text Block] Schedule of Intangible Assets and Goodwill [Table Text Block] Interest-bearing deposits Noninterest-bearing deposits us-gaap_LoansAndLeasesReceivableRelatedParties Loans and Leases Receivable, Related Parties Balance, beginning of year Balance, end of year us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes Sate and local taxes, net of federal benefit Fair Value Measurements, Nonrecurring [Table Text Block] Debt Securities, Held-to-Maturity, Credit Quality Indicator [Table Text Block] Cash flows from financing activities us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate Tax at federal statutory rate fmfg_NumberOfEmployeesCoveredByLifeInsurancePolicy Number of Employees Covered by Life Insurance Policy Number of employees provided with certain life insurance benefits. fmfg_NumberOfEmployeesParticipatingInPlan Number of Employees Participating in Plan Number of employees participating in the plan. fmfg_ExpectedSalesCostsToAppraisedValuePercentage Expected Sales Costs to Appraised Value, Percentage Expected sales costs as a percentage of the appraised value. Pledged Status [Domain] Pledged Status [Axis] us-gaap_IncomeTaxReconciliationNondeductibleExpenseLifeInsurance Appreciation of cash surrender value of life insurance us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostAllowanceForCreditLossExcludingAccruedInterest AFS, allowance for credit losses us-gaap_IncomeTaxReconciliationNondeductibleExpenseOther Other Class of Stock [Axis] Class of Stock [Domain] Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestBeforeAllowanceForCreditLoss Amortized cost, available for sale Debt Securities, Available-for-Sale, Amortized Cost, Excluding Accrued Interest, before Allowance for Credit Loss Financial Service, Other [Member] us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes Sate and local taxes, net of federal benefit fmfg_IncreaseDecreaseInBankOwnedLifeInsuranceCashSurrenderValue Increase in bank owned life insurance cash surrender value The amount of increase (decrease) in bank owned life insurance cash surrender value during the period. us-gaap_DerivativeNotionalAmount Derivative, Notional Amount Debt Securities, Held-to-Maturity, Allowance for Credit Loss, Excluding Accrued Interest HTM, allowance for credit losses Beginning balance Beginning balance us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate Tax at federal statutory rate Hedging Relationship [Domain] Fair Value Hedging [Member] Available for sale securities Debt Securities, Available-for-Sale, Excluding Accrued Interest Securities available for sale, at fair value Available for Sale Fair value Total Foreclosure in settlement of loan receivable The foreclosure of loan receivable fmfg_DeferredTaxAssetsLiabilitiesDerivativeInstruments Derivatives Amount before allocation of valuation allowances of deferred tax asset (liability) attributable to deductible temporary differences from derivative instruments. Securities held to maturity, at amortized cost less allowance for credit losses of $79 and $60 HTM, net Income Tax Disclosure [Text Block] Hedging Relationship [Axis] Hedging Designation [Axis] Hedging Designation [Domain] Designated as Hedging Instrument [Member] us-gaap_DebtSecuritiesHeldToMaturityExcludingAccruedInterestBeforeAllowanceForCreditLoss Held to maturity securities Debt Securities, Held-to-Maturity, Excluding Accrued Interest, before Allowance for Credit Loss Pass [Member] Held to maturity us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesDebt Available for sale Internal Credit Assessment [Axis] Available for sale Proceeds from Maturities, Prepayments and Calls of Debt Securities, Available-for-Sale Internal Credit Assessment [Domain] Schedule of Nonvested Restricted Stock Units Activity [Table Text Block] us-gaap_ProceedsFromSaleOfOtherRealEstate Proceeds from sale of REO property us-gaap_EquitySecuritiesFvNiCurrentAndNoncurrent Equity security us-gaap_PaymentsToAcquireAvailableForSaleSecuritiesDebt Available for sale us-gaap_PaymentsToAcquireHeldToMaturitySecurities Held to maturity Financial Asset, Past Due [Member] Retirement Plan Name [Axis] Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest Allowance for loan losses Allowance for loan losses fmfg_FinancingReceivableNumberOfLoansNonaccrualStatus Financing Receivable, Number of Loans, Nonaccrual Status Represents the number of loans in financing receivables that are on nonaccrual status as of the balance sheet date. Retirement Plan Name [Domain] Financial Asset, Not Past Due [Member] Goodwill and Intangible Assets Disclosure [Text Block] Goodwill [Table] Loans, less allowance for credit losses of $4,361 and $4,260 Financing Receivable, Excluding Accrued Interest, after Allowance for Credit Loss Retirement Plan Type [Axis] Loans Loans Loans Retirement Plan Type [Domain] Credit Facility [Axis] Supplemental Employee Retirement Plan [Member] Credit Facility [Domain] us-gaap_FinancingReceivableNonaccrualInterestIncome Financing Receivable, Nonaccrual, Interest Income Deferred origination fees net of costs us-gaap_FinancingReceivableExcludingAccruedInterestYearFiveOriginatedFourYearsBeforeCurrentFiscalYear Pass Special Mention [Member] us-gaap_FinancingReceivableExcludingAccruedInterestYearFourOriginatedThreeYearsBeforeCurrentFiscalYear Pass Substandard [Member] us-gaap_FinancingReceivableExcludingAccruedInterestYearOneOriginatedCurrentFiscalYear Pass Doubtful [Member] us-gaap_FinancingReceivableExcludingAccruedInterestYearThreeOriginatedTwoYearsBeforeCurrentFiscalYear Pass Nonaccrual Loans [Member] Represents nonaccrual loans. us-gaap_PaymentsToAcquireProductiveAssets Purchases of premises, equipment and software us-gaap_FinancingReceivableExcludingAccruedInterestOriginatedMoreThanFiveYearsBeforeCurrentFiscalYear Pass us-gaap_FinancingReceivableExcludingAccruedInterestRevolving Pass us-gaap_FinancingReceivableExcludingAccruedInterestYearTwoOriginatedFiscalYearBeforeCurrentFiscalYear Pass Effect of dilutive restricted stock units (in shares) Supplemental disclosure of non-cash transactions: Fair value adjustment of equity security Unrealized (gain) loss on equity security Cash paid during the period for income taxes Total taxes paid Collateral Dependent Loans [Member] Represents collateral dependent loans. us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 Total Supplemental disclosure of cash flow information: us-gaap_LongTermDebtFairValue Long-term debt, net Loan Restructuring Modification [Axis] Loan Restructuring Modification [Domain] Extended Maturity [Member] Accretion Amount of accretion recognized for the excess of a loan's cash flows expected to be collected over the investor's initial investment in acquired loans not accounted for as debt securities, with evidence of deterioration of credit quality. Financing Receivable, Modified [Table Text Block] Core Deposits [Member] fmfg_FinancingReceivableModificationsNumberOfContracts Financing Receivable, Modifications, Number of Contracts Number of financing receivables that have been modified by troubled debt restructurings. fmfg_LoansAcquiredAccretableYield Loans Acquired, Accretable Yield Amount of the excess of a loan's cash flows expected to be collected over the investor's initial investment in acquired loans not accounted for as debt securities with evidence of deterioration of credit quality. Collateral Dependent Loans [Table Text Block] Tabular disclosure of collateral dependent loans. Finite-Lived Intangible Assets by Major Class [Axis] Finite-Lived Intangible Assets, Major Class Name [Domain] Non-cash compensation Amount of noncash compensation. Not Rated [Member] Represents not rated. Effective Income Tax Rate Reconciliation [Table] us-gaap_AdvancesFromFederalHomeLoanBanks Federal Home Loan Bank of Atlanta advances fmfg_DebtSecuritiesAvailableforSaleExcludingAccruedInterestNet AFS, net Amount excluding accrued interest, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), net. fmfg_LoansAcquiredOutstandingBalance Loans Acquired, Outstanding Balance The undiscounted sum of all amounts, including amounts deemed principal, interest, fees, penalties, and other under the loan, owed to the investor at the reporting date, whether or not currently due and whether or not any such amounts have been written or charged off by the investor. This relates to loans not accounted for as debt securities, with evidence of deterioration of credit quality since origination that were acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable. fmfg_CollateralDependentLoansFairValueDisclosure Collateral-dependent loans, net Fair value portion of collateral dependent loans. Net Income Net income Net income Financial Instrument Performance Status [Axis] Financial Instrument Performance Status [Domain] Nonperforming Financial Instruments [Member] Proceeds from insurance Restricted Stock Units (RSUs) [Member] Financing Receivable Credit Quality Indicators [Table Text Block] us-gaap_PaymentsToFundLongtermLoansToRelatedParties Payments to Fund Long-Term Loans to Related Parties Schedule of Components of Lease Assets and Liabilities [Table Text Block] Tabular disclosure of components of lease assets and liabilities. Financial Asset, 30 to 59 Days Past Due [Member] us-gaap_ParticipatingMortgageLoansParticipationLiabilitiesAmount Participating Mortgage Loans, Participation Liabilities, Amount Financial Asset, 60 to 89 Days Past Due [Member] Financial Asset, Equal to or Greater than 90 Days Past Due [Member] Financing Receivable, Past Due [Table Text Block] Financial Asset, Aging [Axis] Financial Asset, Aging [Domain] Long-Term Debt, Type [Axis] Long-Term Debt, Type [Domain] us-gaap_PaymentsForProceedsFromFederalHomeLoanBankStock Purchase of stock in FHLB of Atlanta us-gaap_RelatedPartyDepositLiabilities Related Party Deposit Liabilities Cash Provided by (Used in) Investing Activity, Including Discontinued Operation [Abstract] Earnings Per Share, Basic, by Common Class, Including Two-Class Method [Table] us-gaap_IncreaseDecreaseInOtherOperatingLiabilities Other liabilities Related Party Transaction [Table] fmfg_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesRealizedLosses Reclassification adjustment for realized loss Amount of realized losses of reclassification adjustment from accumulated other comprehensive income for unrealized loss realized upon the sale of available-for-sale securities. Accrued interest payable Decrease in accrued interest payable Related Party Transactions Disclosure [Text Block] Amortization (accretion) of right of use asset Amount of periodic reduction over lease term of carrying amount of right-of-use asset from operating lease. Schedule of Related Party Transactions [Table Text Block] us-gaap_IncomeTaxPaidFederalAfterRefundReceived Federal us-gaap_IncomeTaxPaidStateAndLocalAfterRefundReceived Maryland fmfg_PolicyTable Subordinated Debt [Member] us-gaap_InterestExpenseOperating Total interest expense us-gaap_AllocatedShareBasedCompensationExpense Share-Based Payment Arrangement, Expense Deferred loan fees and costs, net us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment Outstanding loan balances evaluated for impairment individually us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment Outstanding loan balances evaluated for impairment collectively us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1 Allowance for loan losses ending balance evaluated for impairment individually us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment Allowance for loan losses ending balance evaluated for impairment collectively us-gaap_ComprehensiveIncomeNetOfTax Total comprehensive income us-gaap_DebtInstrumentPeriodicPayment Debt Instrument, Periodic Payment fmfg_IncreaseDecreaseInMortgageLoansHeldforsaleExcludingCollectionOfMortgageLoansHeldforsale Mortgage loans held for sale Represents increase (decrease) in mortgage loans held-for-sale excluding collection of mortgage loans held-for-sale. Credit Loss Status [Axis] us-gaap_OffBalanceSheetCreditLossLiabilityCreditLossExpenseReversal Provision for credit losses Credit Loss Status [Domain] Financial Asset Acquired with Credit Deterioration [Member] us-gaap_OffBalanceSheetCreditLossLiability Beginning balance Ending balance us-gaap_SharesOutstanding Balance (in shares) Balance (in shares) Common Stock, Shares, Outstanding (in shares) us-gaap_DebtWeightedAverageInterestRate Securites sold under repurchase agreements Financing Receivable, Allowance for Credit Loss [Table Text Block] us-gaap_IncreaseDecreaseInOtherOperatingAssets Other assets us-gaap_DebtInstrumentInterestRateDuringPeriod Securites sold under repurchase agreements Dividends reinvested The amount of adjustments related to capital dividends for reinvested adjustment. us-gaap_DebtInstrumentInterestRateStatedPercentage Debt Instrument, Interest Rate, Stated Percentage us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent Operating Lease, Weighted Average Discount Rate, Percent us-gaap_OperatingLeaseCost Operating Lease, Cost Additions to right of use assets obtained in exchange for lease liabilities us-gaap_OperatingLeaseWeightedAverageRemainingLeaseTerm1 Operating Lease, Weighted Average Remaining Lease Term (Year) Cash dividends, $0.67 per share Cash dividends, $0.67 per share us-gaap_DebtInstrumentFaceAmount Debt Instrument, Face Amount Capital Debt Instrument [Axis] Debt Instrument, Name [Domain] us-gaap_IncreaseDecreaseInAccruedInterestReceivableNet Accrued interest receivable fmfg_PurchasedLoansAmortizableYield Beginning balance Ending balance Amount of amortizable yield on purchased loans. us-gaap_IncreaseDecreaseInIncomeTaxesReceivable Change in income tax receivable Amortizable Yield Premium on Purchased Loans [Table Text Block] Tabular disclosure for amortizable yield premium on purchased loans. Schedule of Long-Term Debt Instruments [Table Text Block] Dividends reinvested (in shares) Vested restricted stock units (in shares) us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity Line of Credit Facility, Maximum Borrowing Capacity Vested restricted stock units Stock-based compensation (in shares) Stock-based compensation fmfg_DeferredTaxAssetsLeaseLiabilityNetOfRightOfUseAsset Lease liability Amount, before allocation of valuation allowance, of deferred tax asset attributable to deductible temporary differences from lease liability, net of right-of-use asset. Line of Credit Facility [Table] Liabilities and Equity us-gaap_LiabilitiesAndStockholdersEquity Total liabilities and stockholders' equity us-gaap_CashUninsuredAmount Cash, Uninsured Amount Reported Value Measurement [Member] Asset Pledged as Collateral [Member] Decrease (increase) in Retained earnings Retained earnings Accumulated other comprehensive loss Debt Disclosure [Text Block] Cash and Cash Equivalent [Table] us-gaap_OperatingLeaseLiability Operating lease liabilities Operating Lease, Liability, Statement of Financial Position [Extensible Enumeration] Cash and Cash Equivalents Disclosure [Text Block] us-gaap_OperatingLeaseRightOfUseAsset Operating lease right of use asset Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration] us-gaap_LesseeOperatingLeaseLiabilityPaymentsDue Total lease payments us-gaap_LesseeOperatingLeaseLiabilityUndiscountedExcessAmount Less imputed interest Deferred tax expense (benefit) us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearThree 2028 2029 2030 us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueAfterYearFive Thereafter us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueNextTwelveMonths 2026 us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearTwo 2027 Stock based compensation Stock based compensation Lessee, Operating Lease, Liability, to be Paid, Maturity [Table Text Block] Depreciation and amortization us-gaap_AssetsFairValueDisclosure Other real estate owned us-gaap_AccretionAmortizationOfDiscountsAndPremiumsInvestments Amortization of premiums and (accretion of discounts), net us-gaap_MortgagesHeldForSaleFairValueDisclosure Mortgage loans held for sale us-gaap_CashSurrenderValueFairValueDisclosure Bank owned life insurance us-gaap_FederalHomeLoanBankBorrowingsFairValueDisclosure Federal Home Loan Bank advances us-gaap_CashAndCashEquivalentsFairValueDisclosure Cash and cash equivalents Increase (decrease) in fair value of interest rate swap agreements us-gaap_SecuritiesLoanedOrSoldUnderAgreementsToRepurchaseFairValueDisclosure Securities sold under repurchase agreements Fair Value, Nonrecurring [Member] Common stock, par value $.01 per share, authorized 5,000,000 shares; issued and outstanding 3,229,795 shares in 2025 and 3,166,653 shares in 2024 Fair Value, Recurring and Nonrecurring [Table] Adjustments to reconcile net income to net cash provided by operating activities Measurement Frequency [Axis] Measurement Frequency [Domain] Fair Value, Recurring [Member] Common Stock, Shares Authorized (in shares) Loans [Member] Common stock, share issued (in shares) Common Stock, Shares, Issued (in shares) Fair Value Measurements, Recurring and Nonrecurring [Table Text Block] Common Stock, Par or Stated Value Per Share (in dollars per share) Statistical Measurement [Domain] Lessee, Lease, Description [Table] us-gaap_OperatingLeasePayments Operating Lease, Payments Maximum [Member] Minimum [Member] Weighted Average [Member] Ownership [Domain] Product and Service [Axis] Product and Service [Domain] Statistical Measurement [Axis] Ownership [Axis] Cash paid during the period for interest Property, Plant and Equipment Disclosure [Text Block] Property, Plant and Equipment [Table Text Block] Equity in undistributed income of insurance subsidiary fmfg_EquityInUndistributedIncomeOfInsuranceSubsidiary The amount of equity in undistributed income of insurance subsidiary. Portion at Fair Value Measurement [Member] Estimate of Fair Value Measurement [Member] fmfg_EquityInUndistributedIncomeOfSubsidiaries Equity in undistributed income of subsidiaries The amount of equity in undistributed income of subsidiaries, including but not limited to bank subsidiary and insurance subsidiary. Measurement Basis [Axis] Federal Home Loan Bank Branch [Domain] Tax Year 2019, 2020 and 2021 [Member] Represents tax year 2019, 2020 and 2021. Fair Value, Inputs, Level 3 [Member] Federal Home Loan Bank of Atlanta [Member] Tax Year 2016, 2017 and 2018 [Member] Represents tax year 216, 2017 and 2018. Fair Value Hierarchy and NAV [Domain] Fair Value, Inputs, Level 1 [Member] Federal Home Loan Bank Branch [Axis] Fair Value, Inputs, Level 2 [Member] fmfg_DeferredTaxAssetTaxDeferredExpenseReserveAndAccrualHeldToMaturitySecuritiesAllowanceForCreditLoss Allowance for credit losses on held to maturity securities Amount, before allocation of valuation allowance, of deferred tax asset attributable to deductible temporary difference from allowance for credit loss on held to maturity securities. fmfg_DeferredTaxLiabilitiesPurchaseAccountingAdjustments Purchase accounting adjustments Amount of deferred tax liability attributable to taxable temporary differences from purchase accounting adjustments. Fair Value Hierarchy and NAV [Axis] Standard & Poor's, AA Rating [Member] Cash Provided by (Used in) Operating Activity, Including Discontinued Operation [Abstract] Standard & Poor's, A Rating [Member] Credit Rating, Standard & Poor's [Axis] External Credit Rating, Standard & Poor's [Domain] fmfg_OtherRealEstateOwnedNet Other real estate owned expense, net Net amount of expense recognized during the period for the foreclosure on real estate assets. Standard & Poor's, AAA Rating [Member] Statement [Line Items] Additional paid-in capital Additional paid-in capital AOCI Attributable to Parent [Member] Stockholders' equity Held-to-Maturity Securities [Member] us-gaap_InvestmentInFederalHomeLoanBankStockFairValueDisclosure Restricted stock, at cost us-gaap_LoansReceivableFairValueDisclosure Loans, net Fair Value, by Balance Sheet Grouping [Table] Fair Value Disclosures [Text Block] Fair Value, by Balance Sheet Grouping [Table Text Block] Liabilities Total liabilities Other Liabilities Other liabilities Schedule of Income Taxes Paid [Table Text Block] The tabular disclosure of income taxes paid. Hedged items us-gaap_ChangeInUnrealizedGainLossOnFairValueHedgingInstruments1 Interest rate contracts designated as hedging instruments Derivative liability Fair value hedge Accrued interest payable us-gaap_InterestPayableCurrentAndNoncurrent Accrued interest payable Parent Company [Member] Consolidated Entities [Axis] Consolidated Entities [Domain] Condensed Cash Flow Statement [Table] Condensed Balance Sheet Statement [Table] Condensed Cash Flow Statement [Table Text Block] Condensed Balance Sheet [Table Text Block] Condensed Income Statement [Table Text Block] fmfg_DeferredTaxAssetTaxDeferredExpenseReserveAndAccrualUnfundedCommitmentsAllowanceForCreditLoss Allowance for credit losses on unfunded commitments Amount, before allocation of valuation allowance, of deferred tax asset attributable to deductible temporary difference from allowance for credit loss on unfunded commitments. Condensed Income Statement [Table] Condensed Financial Statements [Table] fmfg_PaymentsForBankOwnedLifeInsurance Redemption/pay out of BOLI Amount of cash outflow for payment of bank owned life insurance. Retained Earnings [Member] Additional Paid-in Capital [Member] Common Stock [Member] Equity Components [Axis] Equity Component [Domain] Long-term debt Long-term debt, net of unamortized issuance costs Other Assets [Member] dei_DocumentAnnualReport Document Annual Report Statement of Financial Position Location, Balance [Axis] Statement of Financial Position Location, Balance [Domain] dei_EntityIncorporationStateCountryCode Entity Incorporation, State or Country Code dei_DocumentTransitionReport Document Transition Report dei_DocumentFinStmtErrorCorrectionFlag Document Financial Statement Error Correction [Flag] Mortgage loans held for sale dei_EntityInteractiveDataCurrent Entity Interactive Data Current us-gaap_SecuritiesSoldUnderAgreementsToRepurchase Securities sold under repurchase agreements us-gaap_FederalHomeLoanBankAdvancesMaturitiesSummaryDueWithinOneYearOfBalanceSheetDate Federal Home Loan Bank advances mature in dei_Security12gTitle Title of 12(g) Security Hedging liability us-gaap_DerivativeAssetFairValueGrossLiability Name of Property [Axis] Name of Property [Domain] us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense Liability for Claims and Claims Adjustment Expense Statement [Table] us-gaap_InterestExpenseLoansFromOtherFederalHomeLoanBanks Federal Home Loan Bank advances fmfg_LongTermDebtMaturitiesRepaymentsOfPrincipalInTenYears Subordinated Debt (net of issuance costs) matures in Amount of long-term debt payable, sinking fund requirement, and other securities issued that are redeemable by holder at fixed or determinable price and date, maturing in tenth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach). us-gaap_DerivativeAssetsLiabilitiesAtFairValueNet Net fair value hedge Income Statement [Abstract] Other comprehensive loss us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFive Long-Term Debt, Maturity, Year Five fmfg_LongTermDebtMaturitiesRepaymentsOfPrincipalInElevenYears Subordinated Debt (net of issuance costs) matures in Amount of long-term debt payable, sinking fund requirement, and other securities issued that are redeemable by holder at fixed or determinable price and date, maturing in eleventh fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach). us-gaap_Deposits Total deposits Interest-bearing Interest-bearing Deposit Liabilities Noninterest-bearing Accounting Standards Update and Change in Accounting Principle [Table] us-gaap_TimeDeposits Time Deposits Money market NOW Deposit Liabilities, Type [Table Text Block] Savings Total unrealized gain on investment securities available for sale Deposits us-gaap_DerivativeFairValueHedgeIncludedInEffectivenessGainLoss Net gain on fair value hedging relationships included in interest income from investment securities - taxable Loan Origination Commitments [Member] Unused lines of Credit [Member] Long-Term Debt [Member] Equity, Attributable to Parent us-gaap_StockholdersEquity Total Stockholders' equity Balance Balance us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax Unrealized losses, available for sale us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax Unrealized gains, available for sale Interest Rate Swap [Member] us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost Over one to five years us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost Over five to ten years us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost Over ten years