Loans and Leases, Effect of Loan Modifications Made to Borrowers Experiencing Financial Difficulty (Details)	12 Months Ended
Dec. 31, 2023 Loan
Commercial Real Estate [Member]
Effect of Loan Modifications Made to Borrowers Experiencing Financial Difficulty [Abstract]
Number of loans modified	1
Commercial Real Estate [Member] \| Contractual Term Extension [Member]
Effect of Loan Modifications Made to Borrowers Experiencing Financial Difficulty [Abstract]
Contractual term extended	119 months
Residential and Home Equity [Member]
Effect of Loan Modifications Made to Borrowers Experiencing Financial Difficulty [Abstract]
Number of loans modified	1
Residential and Home Equity [Member] \| Contractual Interest Rate Reduction [Member]
Effect of Loan Modifications Made to Borrowers Experiencing Financial Difficulty [Abstract]
Contractual interest rate decreased	1.00%
Residential and Home Equity [Member] \| Contractual Term Extension [Member]
Effect of Loan Modifications Made to Borrowers Experiencing Financial Difficulty [Abstract]
Contractual term extended	120 months
Commercial & Industrial [Member]
Effect of Loan Modifications Made to Borrowers Experiencing Financial Difficulty [Abstract]
Number of loans modified	2
Commercial & Industrial [Member] \| Contractual Term Extension [Member]
Effect of Loan Modifications Made to Borrowers Experiencing Financial Difficulty [Abstract]
Contractual term extended	11 months

No definition available.

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

- Definition

Number of modified loans to borrowers experiencing financial difficult during the period.

+ References

No definition available.

+ Details

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

- Definition

Percentage of decrease in weighted average contractual interest rate of financing receivable modified for debtor experiencing financial difficulty.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 12A
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481933/310-10-55-12A

+ Details

Name:	us-gaap_FinancingReceivableModifiedWeightedAverageInterestRateDecreaseFromModification
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Period of increase in weighted average maturity of financing receivable modified for debtor experiencing financial difficulty, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModifiedWeightedAverageTermIncreaseFromModification
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_ExtendedMaturityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=fmcb_RealEstateResidentialAndHomeEquityPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_ContractualInterestRateReductionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=fmcb_CommercialAndIndustrialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: