Business Combinations (Details) - Delta National Bancorp [Member] $ in Millions	Nov. 18, 2016 USD ($)
Business Acquisition [Line Items]
Loans	$ 32.4
Deposits	103.7
Investments	$ 38.7

Amount of investments in debt and equity securities, including, but not limited to, held-to-maturity, trading and available-for-sale acquired at the acquisition date.

No definition available.

Name:	fmcb_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedInvestmentSecurities
Namespace Prefix:	fmcb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant