Employee Benefit Plans (Details) (USD $)	12 Months Ended
Dec. 31, 2014 Contribution	Dec. 31, 2013	Dec. 31, 2012
Profit Sharing Plan [Abstract]
Requisite service period	1 year
Number of annual employer contribution	2
Employer discretionary contribution amount	$ 875,000	$ 825,000	$ 800,000
Employer mandatory contributions amount	1,000,000	952,000	941,000
Annual vesting percentage, first year (in hundredths)	0.00%
Annual vesting percentage, full year thereafter (in hundredths)	25.00%
Vesting percentage after five years (in hundredths)	100.00%
Executive Retirement Plan and Life Insurance Arrangements [Abstract]
Employer contributions to Non-Qualified Executive Retirement Plan	2,700,000	2,700,000	2,600,000
Accrued benefit liability under Non-Qualified Executive Retirement Plan	27,300,000	24,100,000
Tax-exempt interest on the life insurance policies	1,881,000us-gaap_BankOwnedLifeInsuranceIncome	1,856,000us-gaap_BankOwnedLifeInsuranceIncome	1,836,000us-gaap_BankOwnedLifeInsuranceIncome
Cash surrender value of life insurance policies	54,000,000	52,100,000
Senior Management Retention Plan [Abstract]
Contribution to the Senior Management Retention Plan	$ 475,000	$ 536,000	$ 206,000

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:	na
Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

Name:	fmcb_DefinedContributionPlanEmployersContributionAnnualVestingPercentage
Namespace Prefix:	fmcb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

Name:	fmcb_DefinedContributionPlanEmployersContributionAnnualVestingPercentageAfterFirstYear
Namespace Prefix:	fmcb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

Name:	fmcb_DefinedContributionPlanEmployersContributionAnnualVestingPercentageAfterFiveYear
Namespace Prefix:	fmcb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

The minimum period that the individual is required to perform services in order to participate in the defined contribution plan.

+ References

No definition available.

+ Details

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

- Details

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

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

Accrued benefit liability under the Non-Qualified Executive Retirement Plan.

+ References

No definition available.

+ Details

Name:	fmcb_NonQualifiedExecutiveRetirementPlanAccruedBenefitLiability
Namespace Prefix:	fmcb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Contributions made by the employer to the Non-Qualified Executive Retirement Plan.

+ References

No definition available.

+ Details

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

- Details

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

- Definition

Earnings on or other increases in the value of the cash surrender value of bank owned life insurance policies.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.13)

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

+ Details

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

- Definition

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.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.17)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 17

-Article 5

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section 45

-Paragraph 4

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=28358313&loc=d3e6812-107765

+ Details

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

- Details

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

- Definition

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

+ References

No definition available.

+ Details

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