Investment Securities, Proceeds from Sales and Calls of Securities (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2022	Sep. 30, 2021
Proceeds from Sales and Calls of Securities [Abstract]
Gross proceeds	$ 31,394	$ 301,320
Gross gains	2	5,570
Gross losses	$ 2,987	$ 3,016

This item represents the gross profit realized on the sales and calls of debt or equity securities.

No definition available.

Name:	fmcb_SecuritiesGrossRealizedGains
Namespace Prefix:	fmcb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration