Investment Securities - Proceeds from Sales and Calls of Investment Securities (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Proceeds From Sales And Calls Of Investment Securities Abstract
Gross proceeds	$ 25,797	$ 70,721	$ 39,901
Gross gains	189	839	0
Gross losses	$ 145	$ 96	$ 8,199

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