Regulatory Matters (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Bancorp
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital, Actual, Amount	$ 287,435	$ 247,453
Total Capital (to risk-weighted assets), Actual, Ratio	14.91%	14.32%
Total Capital, Minimum Capital for Capital Adequacy Purposes, Amount	$ 178,323	$ 149,039
Total Capital (to risk-weighted assets), Minimum Capital for Capital Adequacy Purposes, Ratio	9.25%	8.625%
Total Capital, Minimum Capital to be Well Capitalized, Amount	$ 192,782	$ 172,799
Total Capital (to risk-weighted assets), Minimum to be Well Capitalized, Ratio	10.00%	10.00%
Tier 1 Risk Based Capital, Actual, Amount	$ 270,710	$ 231,111
Tier 1 Risk Based Capital (to risk-weighted assets), Actual, Ratio	14.04%	13.37%
Tier 1 Capital, Minimum Capital for Capital Adequacy Purposes, Amount	$ 139,767	$ 114,479
Tier 1 Risk Based Capital (to risk-weighted assets), Minimum Capital for Capital Adequacy Purposes, Ratio	7.25%	6.625%
Tier 1 Risk Based Capital, Minimum Capital to be Well Capitalized, Amount	$ 154,225	$ 138,239
Tier 1 Risk Based Capital (to risk-weighted assets), Minimum Capital to be Well Capitalized, Ratio	8.00%	8.00%
Tier 1 Leverage Capital, Amount, Actual	$ 270,710	$ 231,111
Tier 1 Leverage Capital (to average assets), Actual, Ratio	12.13%	11.39%
Tier 1 Leverage Capital, Minimum Capital for Capital Adequacy Purposes, Amount	$ 89,285	$ 81,189
Tier 1 Leverage Capital (to average assets), Minimum Capital for Capital Adequacy Purposes, Ratio	4.00%	4.00%
Tier 1 Leverage Capital, Actual, Amount	$ 111,607	$ 101,486
Tier 1 Leverage Capital (to average assets), Minimum Capital to be Well Capitalized, Ratio	5.00%	5.00%
Tier 1 Common Equity	$ 265,119	$ 225,925
Tier 1 Common Equity To Average Assets	13.75%	13.07%
Tier 1 Common Equity Required for Capital Adequacy	$ 110,849	$ 88,559
Tier 1 Common Equity Required for Capital Adequacy to Average Assets	5.75%	5.125%
Tier 1 Common Equity Required to be Well Capitalized	$ 125,308	$ 112,319
Tier 1 Common Equity Required to be Well Capitalized to Average Assets	6.50%	6.50%
The Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital, Actual, Amount	$ 283,885	$ 243,468
Total Capital (to risk-weighted assets), Actual, Ratio	14.73%	14.09%
Total Capital, Minimum Capital for Capital Adequacy Purposes, Amount	$ 178,281	$ 149,016
Total Capital (to risk-weighted assets), Minimum Capital for Capital Adequacy Purposes, Ratio	9.25%	8.625%
Total Capital, Minimum Capital to be Well Capitalized, Amount	$ 192,737	$ 172,772
Total Capital (to risk-weighted assets), Minimum to be Well Capitalized, Ratio	10.00%	10.00%
Tier 1 Risk Based Capital, Actual, Amount	$ 267,160	$ 222,127
Tier 1 Risk Based Capital (to risk-weighted assets), Actual, Ratio	13.86%	13.15%
Tier 1 Capital, Minimum Capital for Capital Adequacy Purposes, Amount	$ 139,734	$ 114,462
Tier 1 Risk Based Capital (to risk-weighted assets), Minimum Capital for Capital Adequacy Purposes, Ratio	7.25%	6.625%
Tier 1 Risk Based Capital, Minimum Capital to be Well Capitalized, Amount	$ 154,189	$ 138,218
Tier 1 Risk Based Capital (to risk-weighted assets), Minimum Capital to be Well Capitalized, Ratio	8.00%	8.00%
Tier 1 Leverage Capital, Amount, Actual	$ 267,160	$ 222,127
Tier 1 Leverage Capital (to average assets), Actual, Ratio	11.97%	11.19%
Tier 1 Leverage Capital, Minimum Capital for Capital Adequacy Purposes, Amount	$ 89,275	$ 81,176
Tier 1 Leverage Capital (to average assets), Minimum Capital for Capital Adequacy Purposes, Ratio	4.00%	4.00%
Tier 1 Leverage Capital, Actual, Amount	$ 111,593	$ 101,469
Tier 1 Leverage Capital (to average assets), Minimum Capital to be Well Capitalized, Ratio	5.00%	5.00%
Tier 1 Common Equity	$ 267,160	$ 222,127
Tier 1 Common Equity To Average Assets	13.86%	13.15%
Tier 1 Common Equity Required for Capital Adequacy	$ 110,824	$ 88,546
Tier 1 Common Equity Required for Capital Adequacy to Average Assets	5.75%	5.125%
Tier 1 Common Equity Required to be Well Capitalized	$ 125,279	$ 112,302
Tier 1 Common Equity Required to be Well Capitalized to Average Assets	6.50%	6.50%

Tier One Common Equity

No definition available.

Name:	bmrc_TierOneCommonEquity
Namespace Prefix:	bmrc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Tier One Common Equity Required for Capital Adequacy

+ References

No definition available.

+ Details

Name:	bmrc_TierOneCommonEquityRequiredforCapitalAdequacy
Namespace Prefix:	bmrc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Tier One Common Equity Required for Capital Adequacy to Average Assets

+ References

No definition available.

+ Details

Name:	bmrc_TierOneCommonEquityRequiredforCapitalAdequacytoAverageAssets
Namespace Prefix:	bmrc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Tier One Common Equity Required to be Well Capitalized

+ References

No definition available.

+ Details

Name:	bmrc_TierOneCommonEquityRequiredtobeWellCapitalized
Namespace Prefix:	bmrc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Tier One Common Equity Required to be Well Capitalized to Average Assets

+ References

No definition available.

+ Details

Name:	bmrc_TierOneCommonEquityRequiredtobeWellCapitalizedtoAverageAssets
Namespace Prefix:	bmrc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Tier One Common Equity To Average Assets

+ References

No definition available.

+ Details

Name:	bmrc_TierOneCommonEquityToAverageAssets
Namespace Prefix:	bmrc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The amount of total capital as defined in the regulations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 948
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6490092&loc=d3e47304-110998

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1H
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1H
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 948
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=6490092&loc=d3e47080-110998

+ Details

Name:	us-gaap_Capital
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The minimum amount of Total Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1H
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826

+ Details

Name:	us-gaap_CapitalRequiredForCapitalAdequacy
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The minimum Total risk based capital ratio (total capital divided by risk weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1H
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958570-112826

+ Details

Name:	us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The amount of Total Capital required to be categorized as well as capitalized under the regulatory framework for prompt corrective action.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826

+ Details

Name:	us-gaap_CapitalRequiredToBeWellCapitalized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The Total risk based capital ratio (total capital divided by risk weighted assets) required to be categorized as "well capitalized", in the regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Total Capital divided by risk weighted assets as defined in the regulations.

+ References

+ Details

Name:	us-gaap_CapitalToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

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

- Definition

Tier 1 Leverage Capital as defined in the regulations. Also called Core Capital.

+ References

+ Details

Name:	us-gaap_TierOneLeverageCapital
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The minimum amount of Tier 1 Leverage Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacy
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The minimum amount of Tier 1 Leverage Capital for capital adequacy purposes divided by average assets as defined in the regulations.

+ References

+ Details

Name:	us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The amount of Tier 1 Leverage Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The amount of Tier 1 Leverage Capital required to be categorized as well capitalized divided by average assets as defined in the regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalizedToAverageAssets
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Tier 1 Leverage Capital divided by average assets as defined in the regulations.

+ References

+ Details

Name:	us-gaap_TierOneLeverageCapitalToAverageAssets
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Tier 1 Risk Based Capital as defined in the regulations.

+ References

+ Details

Name:	us-gaap_TierOneRiskBasedCapital
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The minimum amount of Tier 1 Risk Based Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacy
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The minimum Tier One Capital Ratio (Tier one capital divided by risk-weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958570-112826

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1H
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826

+ Details

Name:	us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The amount of Tier 1 Risk Based Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The Tier 1 capital ratio (Tier 1 capital divided by risk weighted assets) required to be categorized as "well capitalized" under the regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Tier 1 capital divided by risk weighted assets as defined by regulations.

+ References

+ Details

Name:	us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_ConsolidatedEntitiesAxis=us-gaap_ParentCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConsolidatedEntitiesAxis=us-gaap_SubsidiariesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: