Significant Accounting Policies, Investment Securities (Details)	3 Months Ended
Mar. 31, 2021 Component
Investment Securities [Abstract]
Number of components into which amount of impairment is split	2

Represents the number of components into which amount of impairment is split.

No definition available.

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