Investment Securities, Proceeds from Sales and Calls of Securities (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2022	Mar. 31, 2021
Proceeds from Sales and Calls of Securities [Abstract]
Gross proceeds	$ 2,190	$ 63,788
Gross gains	0	1,840
Gross losses	$ 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