Investment Securities, Securities in Continuous Unrealized Loss Position (Details)	Mar. 31, 2022 Security
Investment Securities, Qualitative Disclosure [Abstract]
Number of investment securities held	585
Less than 12 months, number of positions	102
12 months or more, number of positions	100

Represents the number of investment positions in investment securities in a continuous unrealized loss position for less than one year or normal operating cycle, if longer, for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

No definition available.

Name:	fmcb_InvestmentSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
Namespace Prefix:	fmcb_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant