Investment Securities - Proceeds from Sales and Calls of Investment Securities (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2025	Sep. 30, 2024
Investment Securities [Abstract]
Gross proceeds	$ 925	$ 70,251
Gross gains	0	839
Gross losses	$ 0	$ 96

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