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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

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- Definition Deposits Money Market Deposits Demand Deposit Marketplace + References No definition available. + Details
Name: | bmrc_DepositsMoneyMarketDepositsDemandDepositMarketplace
Namespace Prefix: | bmrc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Deposits Money Market Deposits Demand Deposit Marketplace One Way + References No definition available. + Details
Name: | bmrc_DepositsMoneyMarketDepositsDemandDepositMarketplaceOneWay
Namespace Prefix: | bmrc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Deposits, Money Market Deposits, Insured Cash Sweep (ICS) + References No definition available. + Details
Name: | bmrc_DepositsMoneyMarketDepositsInsuredCashSweepICS
Namespace Prefix: | bmrc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Deposits Money Market Deposits Insured Cash Sweep ICS One Way + References No definition available. + Details
Name: | bmrc_DepositsMoneyMarketDepositsInsuredCashSweepICSOneWay
Namespace Prefix: | bmrc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition One Way [Abstract] + References No definition available. + Details
Name: | bmrc_OneWayAbstract
Namespace Prefix: | bmrc_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Reciprocal [Abstract] + References No definition available. + Details
Name: | bmrc_ReciprocalAbstract
Namespace Prefix: | bmrc_
Data Type: | xbrli:stringItemType
Balance Type: | na
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- Definition 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. + References No definition available. + Details
Name: | bmrc_TimeDepositsCertificateOfDepositAccountRegistryServiceCDARS
Namespace Prefix: | bmrc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Time Deposits, Certificate Of Deposit Account Registry Service (CDARS), One-Way + References No definition available. + Details
Name: | bmrc_TimeDepositsCertificateOfDepositAccountRegistryServiceCDARSOneWay
Namespace Prefix: | bmrc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Total Network deposits One Way + References No definition available. + Details
Name: | bmrc_TotalNetworkdepositsOneWay
Namespace Prefix: | bmrc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Total Network deposits Reciprocal + References No definition available. + Details
Name: | bmrc_TotalNetworkdepositsReciprocal
Namespace Prefix: | bmrc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant