Investment Securities, Proceeds and Gross Realized Gains and Losses from Sales and Calls of Investment Securities (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Proceeds and Gross Realized Gains and Losses from Sales and Calls of Investment Securities [Abstract]
Gross proceeds	$ 39,901	$ 51,359	$ 301,320
Gross gains	0	2	5,570
Gross losses	$ 8,199	$ 10,691	$ 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