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Summary of Significant Accounting Policies - Investments in Low Income Housing Tax Credit Funds (Details) | 12 Months Ended
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Dec. 31, 2020
Minimum |
Other Ownership Interests [Line Items] |
Ownership percentage in limited partnerships related to Low Income Housing Tax Credits | 1.00%
Maximum |
Other Ownership Interests [Line Items] |
Ownership percentage in limited partnerships related to Low Income Housing Tax Credits | 3.50%

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- Definition Affordable Housing Tax Credits, Investment, Ownership Percentage + References No definition available. + Details
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Namespace Prefix: | bmrc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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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
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |