Investment Securities, Proceeds from Sales (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Proceeds from sales and calls of securities [Abstract]
Proceeds from sales and calls of securities	$ 0	$ 0

The cash inflow associated with the sales and calls of debt and equity securities.

No definition available.

Name:	fmcb_ProceedsFromSalesAndCallsOfSecurities
Namespace Prefix:	fmcb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration