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

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- 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. + Details
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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 + Details
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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 + Details
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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 + Details
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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 + Details
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