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Summary of Significant Accounting Policies - Investments in Low Income Housing Tax Credit Funds (Details)	12 Months Ended
	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%

X

- 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

X

- 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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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
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Period Type: