Leases (Details)	3 Months Ended	12 Months Ended
	Dec. 31, 2020 USD ($) Lease	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Operating Leases [Abstract]
Operating lease ROU assets	$ 4,800,000	$ 4,800,000	$ 4,980,000
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:InterestReceivableAndOtherAssets	us-gaap:InterestReceivableAndOtherAssets	us-gaap:InterestReceivableAndOtherAssets
Operating lease liability	$ 4,918,000	$ 4,918,000	$ 5,030,000.00
Operating Lease, Liability, Statement of Financial Position [Extensible List]	fmcb:InterestPayableAndOtherLiabilities	fmcb:InterestPayableAndOtherLiabilities	fmcb:InterestPayableAndOtherLiabilities
Operating lease cost		$ 833,000	$ 836,000
Number of leases modified | Lease	1
Additional operating lease ROU assets resulting from modification of lease term and payment	$ 542,000	542,000
Additional operating lease liabilities resulting from modification of lease term and payment	$ 542,000	$ 542,000
Minimum [Member]
Operating Leases [Abstract]
Remaining lease term	1 year	1 year
Lease extension option term	5 years	5 years
Maximum [Member]
Operating Leases [Abstract]
Remaining lease term	10 years	10 years
Lease extension option term	10 years	10 years