Deposits - Composition of Network Deposits (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Reciprocal [Abstract]
CDARS	$ 7,622	$ 5,011
ICS	0	56,681
DDM	30,544	41,636
Total network deposits	38,166	103,328
One Way [Abstract]
CDARS	2,434	7,453
ICS	110,929	27,220
DDM	60,000	0
Total network deposits	$ 173,363	$ 34,673

Certificate of Deposit Account Registry Service, a special type of certificate of deposit whereby the participating bank accepts deposits in excess of the FDIC insured maximum and places the deposits through a network to member banks in the CDARS(R) network in increments of less than the FDIC insured maximum to provide the depositor full FDIC insurance coverage.

No definition available.

Name:	bmrc_TimeDepositsCertificateOfDepositAccountRegistryServiceCDARS
Namespace Prefix:	bmrc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant