Investment Securities, Proceeds from Sales (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Proceeds from sales and calls of securities [Abstract]
Proceeds	$ 50,620	$ 2,780	$ 53,620	$ 4,090
Gains	0	1	13	1
Losses	$ 0	$ 0	$ 0	$ 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