Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Available-for-Sale Securities [Abstract]
Fair/Book Value, Total	$ 421,765	$ 463,177	$ 421,765	$ 463,177	$ 417,991
Recurring [Member]
Available-for-Sale Securities [Abstract]
Government Agency & Government-Sponsored Entities	28,643	46,971	28,643	46,971	26,823
Obligations of States and Political Subdivisions		5,704		5,704	5,665
Mortgage Backed Securities	342,077	389,812	342,077	389,812	352,772
Corporate Securities	49,761	14,864	49,761	14,864	22,558
Other	1,284	5,826	1,284	5,826	10,173
Fair/Book Value, Total	421,765	463,177	421,765	463,177	417,991
Recurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Available-for-Sale Securities [Abstract]
Government Agency & Government-Sponsored Entities	23,587	10,627	23,587	10,627	21,731
Obligations of States and Political Subdivisions		0		0	0
Mortgage Backed Securities	0	0	0	0	0
Corporate Securities	8,171	0	8,171	0	4,020
Other	974	5,516	974	5,516	9,863
Fair/Book Value, Total	32,732	16,143	32,732	16,143	35,614
Recurring [Member] \| Other Observable Inputs (Level 2) [Member]
Available-for-Sale Securities [Abstract]
Government Agency & Government-Sponsored Entities	5,056	36,344	5,056	36,344	5,092
Obligations of States and Political Subdivisions		0		0	0
Mortgage Backed Securities	342,077	389,812	342,077	389,812	352,772
Corporate Securities	41,590	14,864	41,590	14,864	18,538
Other	310	310	310	310	310
Fair/Book Value, Total	389,033	441,330	389,033	441,330	376,712
Recurring [Member] \| Significant Unobservable Inputs (Level 3) [Member]
Available-for-Sale Securities [Abstract]
Government Agency & Government-Sponsored Entities	0	0	0	0	0
Obligations of States and Political Subdivisions		5,704		5,704	5,665
Mortgage Backed Securities	0	0	0	0	0
Corporate Securities	0	0	0	0	0
Other	0	0	0	0	0
Fair/Book Value, Total	0	5,704	0	5,704	5,665
Changes in level 3 assets measured at fair value on a recurring basis [Roll Forward]
Balance at Beginning of Period	5,612	5,724	5,665	5,782
Total Realized and Unrealized Gains/(Losses) Included in Income	0	0	0	0
Total Unrealized Gains/(Losses) Included in Other Comprehensive Income	0	0	0	0
Purchase of Securities	0	0	0	0
Sales, Maturities, and Calls of Securities	(31)	(20)	(84)	(78)
Net Transfers out of Available for Sale Securities	(5,581)	0	(5,581)	0
Balance at End of Period	0	5,704	0	5,704
Nonrecurring [Member]
Impaired Loans [Abstract]
Residential 1st Mortgage	477	200	477	200	235
Home Equity Lines and Loans	293	466	293	466	462
Agricultural	366		366		1,010
Commercial	228		228
Total Impaired Loans	1,364	666	1,364	666	1,707
Other Real Estate [Abstract]
Real Estate Construction	2,398	2,553	2,398	2,553	2,553
Agricultural Real Estate	1,268		1,268
Total Other Real Estate	3,666	2,553	3,666	2,553	2,553
Total Assets Measured at Fair Value On a Non-Recurring Basis	5,030	3,219	5,030	3,219	4,260
Nonrecurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Impaired Loans [Abstract]
Residential 1st Mortgage	0	0	0	0	0
Home Equity Lines and Loans	0	0	0	0	0
Agricultural	0		0		0
Commercial	0		0
Total Impaired Loans	0	0	0	0	0
Other Real Estate [Abstract]
Real Estate Construction	0	0	0	0	0
Agricultural Real Estate	0		0
Total Other Real Estate	0	0	0	0	0
Total Assets Measured at Fair Value On a Non-Recurring Basis	0	0	0	0	0
Nonrecurring [Member] \| Other Observable Inputs (Level 2) [Member]
Impaired Loans [Abstract]
Residential 1st Mortgage	0	0	0	0	0
Home Equity Lines and Loans	0	0	0	0	0
Agricultural	0		0		0
Commercial	0		0
Total Impaired Loans	0	0	0	0	0
Other Real Estate [Abstract]
Real Estate Construction	0	0	0	0	0
Agricultural Real Estate	0		0
Total Other Real Estate	0	0	0	0	0
Total Assets Measured at Fair Value On a Non-Recurring Basis	0	0	0	0	0
Nonrecurring [Member] \| Significant Unobservable Inputs (Level 3) [Member]
Impaired Loans [Abstract]
Residential 1st Mortgage	477	200	477	200	235
Home Equity Lines and Loans	293	466	293	466	462
Agricultural	366		366		1,010
Commercial	228		228
Total Impaired Loans	1,364	666	1,364	666	1,707
Other Real Estate [Abstract]
Real Estate Construction	2,398	2,553	2,398	2,553	2,553
Agricultural Real Estate	1,268		1,268
Total Other Real Estate	3,666	2,553	3,666	2,553	2,553
Total Assets Measured at Fair Value On a Non-Recurring Basis	$ 5,030	$ 3,219	$ 5,030	$ 3,219	$ 4,260

Fair value disclosure of loans and lines of credit made to farmers to finance agricultural production.

No definition available.

Name:	fmcb_AgriculturalFairValueDisclosure
Namespace Prefix:	fmcb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value disclosure of other real estate related to Agricultural Other Real Estate.

+ References

No definition available.

+ Details

Name:	fmcb_AgriculturalOtherRealEstate
Namespace Prefix:	fmcb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value disclosure of secured and unsecured loans for purposes such as seasonal working capital needs, inventory financing, equipment purchases and acquisitions.

+ References

No definition available.

+ Details

Name:	fmcb_CommercialLoansFairValueDisclosure
Namespace Prefix:	fmcb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

+ References

No definition available.

+ Details

Name:	fmcb_CorporateSecuritiesFairValueDisclosure
Namespace Prefix:	fmcb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. Government Agencies Debentures, notes and other debt securities issued by US government agencies, for example but not limited to, Government National Mortgage Association (GNMA or Ginnie Mae) which have been categorized as available-for-sale. Excludes US treasury Securities. Government Sponsored Entities (GESs) Debentures, bonds and other debt securities issued by US government sponsored entities, for example, but not limited to, Federal Home Loan Mortgage Corporation (FHLMC or Freddie Mac), Federal National Mortgage Association (FNMA or Fannie Mae), and the Federal Home Loan Bank (FHLB) which have been categorized as available-for-sale.

+ References

No definition available.

+ Details

Name:	fmcb_GovernmentAgencyGovernmentSponsoredEntitiesFairValueDisclosure
Namespace Prefix:	fmcb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value disclosure of revolving, open-end loan extended under a line of credit and secured by the borrower's residential property.

+ References

No definition available.

+ Details

Name:	fmcb_HomeEquitiesLinesAndLoansFairValueDisclosure
Namespace Prefix:	fmcb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents impaired loan receivable as presented on the statement of financial position which may include mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts, or any other loans which are due the Company as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	fmcb_ImpairedLoansFairValueDisclosure
Namespace Prefix:	fmcb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

+ References

No definition available.

+ Details

Name:	fmcb_ObligationOfStatesAndPoliticalSubdivisionsFairValueDisclosure
Namespace Prefix:	fmcb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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

- Definition

Fair value disclosure of other real estate related to construction.

+ References

No definition available.

+ Details

Name:	fmcb_RealEstateConstructionsFairValueDisclosure
Namespace Prefix:	fmcb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value disclosure of loans to purchase or refinance residential real estate for example, but not limited to a home, in which the real estate itself serves as collateral for the loan.

+ References

No definition available.

+ Details

Name:	fmcb_Residential1StMortgageFairValueDisclosure
Namespace Prefix:	fmcb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Total fair value disclosure of other real estate assets.

+ References

No definition available.

+ Details

Name:	fmcb_TotalOtherRealEstateFairValueDisclosure
Namespace Prefix:	fmcb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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

- Definition

This element represents the aggregate of the assets reported on the balance sheet at period end measured at fair value on a nonrecurring basis by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.

+ References

No definition available.

+ Details

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

- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 5

-Subparagraph (aa)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

+ Details

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

- Details

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

- Definition

Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of gain (loss) recognized in other comprehensive income (loss) for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(1a)

-URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258

+ Details

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

- Definition

Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(2)

-URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258

+ Details

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

- Definition

Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(2)

-URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258

+ Details

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

- Definition

Amount of transfer of financial instrument classified as an asset into (out of) level 3 of the fair value hierarchy.

+ References

No definition available.

+ Details

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

- Definition

Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258

+ Details

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

- Details

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

- Definition

Fair value of securitized, pay-through debt securities collateralized by real estate mortgage loans classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Mortgage-Backed Securities

-URI http://asc.fasb.org/extlink&oid=6517959

+ Details

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

- Definition

Fair value portion of other assets.

+ References

No definition available.

+ Details

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