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Loans and Allowance for Credit Losses on Loans - Related Party (Details) - Directors, officers, principal shareholders and associates - USD ($) $ in Millions | Sep. 30, 2021 | Dec. 31, 2020
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Related Party Transaction [Line Items] |  |
Related party loans | $ 8.6 | $ 6.4
Undisbursed commitment to related parties | $ 8.6 | $ 9.1

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- Definition For an unclassified balance sheet, reflects the carrying amount of unpaid loan amounts due from related parties at the balance sheet date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 850 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.7(e)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 + Details
Name: | us-gaap_LoansAndLeasesReceivableRelatedParties
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_RelatedPartyTransactionLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Unused amount of commitments to extend credit to customers. + References No definition available. + Details
Name: | us-gaap_UnusedCommitmentsToExtendCredit
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
Name: | us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=bmrc_DirectorsOfficersPrincipalShareholdersAndAssociatesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |