Fair Value Measurements (Details)	12 Months Ended
Dec. 31, 2020
Impaired Loans [Member]
Fair Value Measurements [Abstract]
Percentage of selling costs	10.00%
ORE [Member]
Fair Value Measurements [Abstract]
Percentage of selling costs	10.00%

Percentage of adjustment for selling cost, used as an input to measure fair value.

No definition available.

Name:	fmcb_FairValueInputsSellingCost
Namespace Prefix:	fmcb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration