Investment Securities, Proceeds from Sales (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Proceeds from sales and calls of securities [Abstract]
Proceeds	$ 53,465	$ 85,433	$ 58,000	$ 95,349
Gains	266	811	272	845
Losses	$ 0	$ 807	$ 0	$ 807

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