Investment Securities, Proceeds from Sales and Calls of Investment Securities (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2024	Sep. 30, 2023
Proceeds from Sales and Calls of Investment Securities [Abstract]
Gross proceeds	$ 70,251	$ 30,482
Gross gains	839	0
Gross losses	$ 96	$ 5,686

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