Federal Home Loan Bank Stock and Other Equity Securities (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Federal Home Loan Bank Stock and Other Equity Securities [Abstract]
FHLB stock and other equity securities	$ 12.7	$ 12.7	$ 12.7

Equity interest in FHLB stock and equity securities classified as other.

No definition available.

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