Deposits (Details) - USD ($)	Sep. 30, 2025	Dec. 31, 2024
Deposits [Abstract]
Overdrawn deposit balances classified as consumer loans	$ 166,000	$ 156,000

Amount of overdrawn time deposit liabilities classified as consumer loans.

No definition available.

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