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Loans and Allowance for Loan Losses - Troubled Debt Restructuring Modifications (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015 loans	Mar. 31, 2014 contract loans
Financing Receivable, Modifications [Line Items]
Number of Contracts Modified		7us-gaap_FinancingReceivableModificationsNumberOfContracts2
Pre-Modification Outstanding Recorded Investment		$ 1,740us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
Post-Modification Outstanding Recorded Investment		1,723us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Post-Modification Outstanding Recorded Investment at period end		1,724bmrc_FinancingReceivableModificationsContractsModifiedDuringPeriodRecordedInvestment
Number of troubled debt restructurings	0us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1	0us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
Commercial and industrial
Financing Receivable, Modifications [Line Items]
Number of Contracts Modified		3us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember
Pre-Modification Outstanding Recorded Investment		1,420us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember
Post-Modification Outstanding Recorded Investment		1,405us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember
Post-Modification Outstanding Recorded Investment at period end		1,405bmrc_FinancingReceivableModificationsContractsModifiedDuringPeriodRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialLoanMember
Home equity
Financing Receivable, Modifications [Line Items]
Number of Contracts Modified		1us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityLoanMember
Pre-Modification Outstanding Recorded Investment		150us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityLoanMember
Post-Modification Outstanding Recorded Investment		150us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityLoanMember
Post-Modification Outstanding Recorded Investment at period end		150bmrc_FinancingReceivableModificationsContractsModifiedDuringPeriodRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityLoanMember
Installment and other consumer
Financing Receivable, Modifications [Line Items]
Number of Contracts Modified		3us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerPortfolioSegmentMember
Pre-Modification Outstanding Recorded Investment		170us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerPortfolioSegmentMember
Post-Modification Outstanding Recorded Investment		168us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerPortfolioSegmentMember
Post-Modification Outstanding Recorded Investment at period end		$ 169bmrc_FinancingReceivableModificationsContractsModifiedDuringPeriodRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerPortfolioSegmentMember

X

- Definition

The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings at end of period.

+ References

No definition available.

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- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 33

-URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953676-111524

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- Definition

Post-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 33

-URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953676-111524

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- Definition

Pre-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 33

-URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953676-111524

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- Definition

Number of financing receivables that have been modified by troubled debt restructurings within the previous 12 months and for which there was a payment default.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 34

-URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953682-111524

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