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Deposits - Stratification of Time Deposits (Details) - USD ($) $ in Thousands | Dec. 31, 2022 | Dec. 31, 2021
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Deposits [Abstract] |  |
Time deposits of less than or equal to $250 thousand | $ 74,421 | $ 96,214
Time deposits of more than $250 thousand | 44,609 | 54,021
Total time deposits | $ 119,030 | $ 150,235

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- Definition The aggregate amount of domestic and foreign time deposit liabilities (including certificates of deposits, individual retirement accounts and open accounts) in denominations of $250,000 or more. + References No definition available. + Details
Name: | bmrc_TimeDeposits250000OrMore
Namespace Prefix: | bmrc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Time Deposits, Less Than Or Equal To $250,000 + References No definition available. + Details
Name: | bmrc_TimeDepositsLessThanOrEqualTo250000
Namespace Prefix: | bmrc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_DepositsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of time deposit liabilities, including certificates of deposit. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 405 -Section 50 -Paragraph 1 -URI https://asc.fasb.org/extlink&oid=116652737&loc=d3e64164-112818 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 210.9-03.12) -URI https://asc.fasb.org/extlink&oid=126897435&loc=d3e534808-122878 + Details
Name: | us-gaap_TimeDeposits
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant