Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013
Available-for-Sale Securities [Abstract]
Fair/Book Value, Total	$ 336,526	$ 404,639	$ 421,765
Bonds Issued by Relationship Banking Customer [Member]
Available-for-Sale Securities [Abstract]
Ownership percentage of investment securities (in hundredths)	100.00%
Recurring [Member]
Available-for-Sale Securities [Abstract]
Government Agency & Government-Sponsored Entities	13,361	28,436	28,643
Mortgage Backed Securities	322,680	324,929	342,077
Corporate Securities		49,380	49,761
Other	485	1,894	1,284
Fair/Book Value, Total	336,526	404,639	421,765
Recurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Available-for-Sale Securities [Abstract]
Government Agency & Government-Sponsored Entities	13,361	23,394	23,587
Mortgage Backed Securities	39,566	0	0
Corporate Securities		8,191	8,171
Other	175	1,584	974
Fair/Book Value, Total	53,102	33,169	32,732
Recurring [Member] \| Other Observable Inputs (Level 2) [Member]
Available-for-Sale Securities [Abstract]
Government Agency & Government-Sponsored Entities	0	5,042	5,056
Mortgage Backed Securities	283,114	324,929	342,077
Corporate Securities		41,189	41,590
Other	310	310	310
Fair/Book Value, Total	283,424	371,470	389,033
Recurring [Member] \| Significant Unobservable Inputs (Level 3) [Member]
Available-for-Sale Securities [Abstract]
Government Agency & Government-Sponsored Entities	0	0	0
Mortgage Backed Securities	0	0	0
Corporate Securities		0	0
Other	0	0	0
Fair/Book Value, Total	0	0	0
Nonrecurring [Member]
Impaired Loans [Abstract]
Commercial Real Estate	92
Residential 1st Mortgages	526	614	477
Home Equity Lines and Loans	763	551	293
Agricultural	363	366	366
Commercial	3,836	820	228
Total Impaired Loans	5,580	2,351	1,364
Other Real Estate [Abstract]
Real Estate Construction	2,441	2,399	2,398
Agricultural Real Estate	858	2,212	1,268
Total Other Real Estate	3,299	4,611	3,666
Total Assets Measured at Fair Value On a Non-Recurring Basis	8,879	6,962	5,030
Nonrecurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Impaired Loans [Abstract]
Commercial Real Estate	0
Residential 1st Mortgages	0	0	0
Home Equity Lines and Loans	0	0	0
Agricultural	0	0	0
Commercial	0	0	0
Total Impaired Loans	0	0	0
Other Real Estate [Abstract]
Real Estate Construction	0	0	0
Agricultural Real Estate	0	0	0
Total Other Real Estate	0	0	0
Total Assets Measured at Fair Value On a Non-Recurring Basis	0	0	0
Nonrecurring [Member] \| Other Observable Inputs (Level 2) [Member]
Impaired Loans [Abstract]
Commercial Real Estate	0
Residential 1st Mortgages	0	0	0
Home Equity Lines and Loans	0	0	0
Agricultural	0	0	0
Commercial	0	0	0
Total Impaired Loans	0	0	0
Other Real Estate [Abstract]
Real Estate Construction	0	0	0
Agricultural Real Estate	0	0	0
Total Other Real Estate	0	0	0
Total Assets Measured at Fair Value On a Non-Recurring Basis	0	0	0
Nonrecurring [Member] \| Significant Unobservable Inputs (Level 3) [Member]
Impaired Loans [Abstract]
Commercial Real Estate	92
Residential 1st Mortgages	526	614	477
Home Equity Lines and Loans	763	551	293
Agricultural	363	366	366
Commercial	3,836	820	228
Total Impaired Loans	5,580	2,351	1,364
Other Real Estate [Abstract]
Real Estate Construction	2,441	2,399	2,398
Agricultural Real Estate	858	2,212	1,268
Total Other Real Estate	3,299	4,611	3,666
Total Assets Measured at Fair Value On a Non-Recurring Basis	$ 8,879	$ 6,962	$ 5,030

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 Real Estate.

+ References

No definition available.

+ Details

Name:	fmcb_AgriculturalRealEstateOtherRealEstate
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

Fair value disclosure of loans solely used for business purposes.

+ References

No definition available.

+ Details

Name:	fmcb_CommercialRealEstateFairValueDisclosure
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

- 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

- Definition

Percent of net assets at close of period. For schedules of investments that are categorized, each category has a percent of net assets for the aggregated value of the Investments in the category.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 12C

-Paragraph 2

-Article 12

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 946

-SubTopic 320

-Section S99

-Paragraph 1

-Subparagraph (SX 210.12-12.3)

-URI http://asc.fasb.org/extlink&oid=34726532&loc=d3e611133-123010

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 946

-SubTopic 320

-Section S99

-Paragraph 4

-Subparagraph (SX 210.12-12C.Column D.2)

-URI http://asc.fasb.org/extlink&oid=34726532&loc=d3e611225-123010

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 12

-Paragraph 3

-Article 12

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 12C

-Paragraph Column D

-Article 12

+ Details

Name:	us-gaap_InvestmentOwnedPercentOfNetAssets
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
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