Investment Securities, Proceeds From Sales (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Proceeds from sales and calls of securities [Abstract]
Gross proceeds	$ 5,080	$ 5,300	$ 99,323
Gross gains	40	1	78
Gross losses	$ 0	$ 0	$ 1,338

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