Summary of Significant Accounting Policies, Loans and Leases Held for Investment (Details)	12 Months Ended
Dec. 31, 2023
Minimum [Member]
Loans and Leases Held for Investment [Abstract]
Period after which loans are placed on non accrual status	90 days

Represents the period after which loans are placed on non accrual status, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

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