Deposits (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Certificates of deposit [Abstract]
Certificates of deposit less than or equal to $250,000	$ 350,304	$ 325,798
Certificates of deposit greater than $250,000	407,048	318,830
Total certificates of deposit	757,352	644,628
Maturities for Certificates of Deposit [Abstract]
2024	294,209
2025	449,006
2026	10,827
2027	2,175
2028	947
Thereafter	188
Total certificates of deposit	757,352	644,628
Overdrawn deposit balances classified as consumer loans	$ 144	$ 149

Amount of overdrawn time deposit liabilities classified as consumer loans.

No definition available.

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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- Definition

Amount of time deposit maturing after fourth fiscal year following current fiscal year. Time deposit includes, but is not limited to, certificate of deposit. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

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

- Definition

The amount of time deposit liabilities, including certificates of deposit in denominations of $250,000 or more.

+ References

No definition available.

+ Details

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

- Definition

The amount of time deposit liabilities, including certificates of deposit in denominations of less than or equal to $250,000.

+ References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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Balance Type:	na
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- Definition

Amount of time deposit maturing in remainder of current fiscal year. Time deposit includes, but is not limited to, certificate of deposit.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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Period Type:	instant

- Definition

Amount of time deposit maturing in fourth fiscal year following current fiscal year. Time deposit includes, but is not limited to, certificate of deposit. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-SubTopic 10
-Topic 470
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481544/470-10-50-1

+ Details

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Balance Type:	credit
Period Type:	instant

- Definition

Amount of time deposit maturing in next fiscal year following current fiscal year. Time deposit includes, but is not limited to, certificate of deposit. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

+ Details

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

- Definition

Amount of time deposit maturing in third fiscal year following current fiscal year. Time deposit includes, but is not limited to, certificate of deposit. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

+ Details

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Period Type:	instant

- Definition

Amount of time deposit maturing in second fiscal year following current fiscal year. Time deposit includes, but is not limited to, certificate of deposit. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

+ Details

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

- Definition

Amount of time deposit liabilities, including certificates of deposit.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-SubTopic 405
-Topic 942
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478353/942-405-50-1

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(12))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1

+ Details

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