Investment Securities, Proceeds from Sales (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Proceeds from sales and calls of securities [Abstract]
Gross proceeds	$ 1,450	$ 50,620	$ 301,320	$ 53,620
Gross gains	0	0	5,570	13
Gross losses	$ 0	$ 0	$ 3,016	$ 0

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