Name:	fmcb_DefinedContributionPlanBenefitVestingPeriod
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Data Type:	xbrli:durationItemType
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Period Type:	duration

Amount of mandatory contributions made by the employer to the Defined Contribution Plan.

No definition available.

Name:	fmcb_DefinedContributionPlanEmployerMandatoryContributionsAmount
Namespace Prefix:	fmcb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Percentage of employer's contributions to a defined contribution plan that vests in a given year.

No definition available.

Name:	fmcb_DefinedContributionPlanEmployersContributionAnnualVestingPercentage
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Data Type:	dtr-types:percentItemType
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Percentage of employer's contributions to a defined contribution plan that vests in after first year.

No definition available.

Name:	fmcb_DefinedContributionPlanEmployersContributionAnnualVestingPercentageAfterFirstYear
Namespace Prefix:	fmcb_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

This element represents the number of annual employer contribution a participant may receive under the plan.

No definition available.

Name:	fmcb_DefinedContributionPlanNumberOfAnnualEmployerContribution
Namespace Prefix:	fmcb_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

The minimum period that the individual is required to perform services in order to participate in the defined contribution plan, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

Name:	fmcb_DefinedContributionPlanRequisiteServicePeriod
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Data Type:	xbrli:durationItemType
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Period Type:	duration

- References

No definition available.

Name:	fmcb_ExecutiveRetirementPlanLifeInsuranceArrangementsAndSeniorManagementRetentionPlanAbstract
Namespace Prefix:	fmcb_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Net investment Gain (Loss) on Non-Qualified Deferred Compensation balance held in a rabbi trust recorded in non-interest income. This line item is offset by an identical entry to non-interest expense resulting in no impact to net income.

No definition available.

Name:	fmcb_NetGainLossNonQualifiedDeferredCompensationPlanInvestmentsNonInterestIncome
Namespace Prefix:	fmcb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Element presents the period after the termination of plan for the account balances to be liquidated and paid out to each participant, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

Name:	fmcb_PeriodForAccountBalancesToBeLiquidatedAndPaidOutToEachParticipant
Namespace Prefix:	fmcb_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

Name:	fmcb_ProfitSharingPlanAbstract
Namespace Prefix:	fmcb_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Carrying amount as of the balance sheet date of amounts which could be received based on the terms of the insurance contract upon surrendering life policies owned by the entity.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(17))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Value of common stock issued to a trust (for example, but not limited to, rabbi trust) set up specifically to accumulate stock for the sole purpose of distribution to participating employees.

No definition available.

Name:	us-gaap_CommonStockSharesHeldInEmployeeTrust
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Number of common stock held in a trust that has been set up specifically to accumulate stock for the sole purpose of distribution to participating employees but not yet earned.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 718
-SubTopic 40
-Section 25
-Paragraph 12
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480637/718-40-25-12

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

Amount of contributions made by employer into fund established for purposes of making future disbursement to individual in accordance with deferred compensation arrangement.

No definition available.

Name:	us-gaap_DeferredCompensationArrangementWithIndividualContributionsByEmployer
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

The carrying amount of the liability as of the balance sheet date to an individual under a deferred compensation arrangement. This amount may be the result of periodic accruals made over the period of active employment, or reflect termination benefits resulting contractual terms or a death benefit.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 710
-SubTopic 10
-Name Accounting Standards Codification
-Section 30
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483043/710-10-30-1

Name:	us-gaap_DeferredCompensationArrangementWithIndividualRecordedLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Amount of discretionary contributions made by an employer to a defined contribution plan.

No definition available.

Name:	us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Amount of operating interest income, including amortization and accretion of premiums and discounts, on securities exempt from state, federal and other income tax.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name Regulation S-K (SK)
-Number 229
-Section 1402
-Paragraph a
-Publisher SEC

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 942
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478524/942-220-S99-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name Regulation S-K (SK)
-Number 229
-Section 1402
-Paragraph b
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-Publisher SEC

Name:	us-gaap_InterestIncomeSecuritiesTaxExempt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- References

No definition available.

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAdditionalGeneralDisclosuresAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Number of shares authorized for issuance under share-based payment arrangement.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)(3)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480429/718-10-50-2

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

Number of shares issued under share-based payment arrangement.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480429/718-10-50-2

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

Name:	us-gaap_PlanNameAxis=fmcb_Two025PlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	srt_TitleOfIndividualAxis=srt_ExecutiveOfficerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	srt_TitleOfIndividualAxis=srt_ManagementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_DeferredCompensationArrangementWithIndividualShareBasedPaymentsByTypeOfDeferredCompensationAxis=us-gaap_DeferredCompensationShareBasedPaymentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: