Investment Securities, Proceeds From Sales (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Proceeds from sales and calls of securities [Abstract]
Proceeds	$ 85,433	$ 28,297	$ 95,349	$ 77,912
Gains	811	285	845	1,189
Losses	$ 807	$ 1,422	$ 807	$ 1,437

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