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Investment Securities - Non-Marketable Securities (Details) (USD $)	3 Months Ended	0 Months Ended
	Mar. 31, 2015	Jan. 28, 2015	Dec. 31, 2014	Apr. 29, 2015
Schedule of Cost-method Investments [Line Items]
Federal Home Loan Bank stock, par value	$ 100bmrc_FederalHomeLoanBankStockParValue
Investments in low income housing tax credit funds	$ 2,800,000us-gaap_AmortizationMethodQualifiedAffordableHousingProjectInvestments		$ 1,800,000us-gaap_AmortizationMethodQualifiedAffordableHousingProjectInvestments
Low income housing tax credits and other tax benefits	72,000bmrc_AmortizationMethodQualifiedAffordableHousingProjectInvestmentsTaxCredits
Low income housing amortization expense	56,000us-gaap_AmortizationMethodQualifiedAffordableHousingProjectInvestmentsAmortization
Unfunded commitments for low income housing tax credit funds	2,200,000bmrc_AffordableHousingProgramUnfundedCommitments
Visa Inc. | Visa Inc. Class A common stock
Schedule of Cost-method Investments [Line Items]
Stockholders' Equity Note, Stock Split, Conversion Ratio		4us-gaap_StockholdersEquityNoteStockSplitConversionRatio1 / dei_LegalEntityAxis = bmrc_VisaIncMember / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember
Visa Inc. | Visa Inc. Class B common stock
Schedule of Cost-method Investments [Line Items]
Stockholders' Equity Note, Stock Split, Conversion Ratio	1.6483us-gaap_StockholdersEquityNoteStockSplitConversionRatio1 / dei_LegalEntityAxis = bmrc_VisaIncMember / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassBMember
Visa Inc. Class B common stock
Schedule of Cost-method Investments [Line Items]
Number of shares of securities carried at cost	16,939us-gaap_InvestmentOwnedBalanceShares / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = bmrc_VisaInc.ClassBCommonStockMember
Carrying value of securities carried at cost	0us-gaap_CostMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = bmrc_VisaInc.ClassBCommonStockMember
Fair value of Class B common stock	1,800,000us-gaap_CostMethodInvestmentsFairValueDisclosure / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = bmrc_VisaInc.ClassBCommonStockMember		1,800,000us-gaap_CostMethodInvestmentsFairValueDisclosure / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = bmrc_VisaInc.ClassBCommonStockMember
Subsequent event
Schedule of Cost-method Investments [Line Items]
Federal Home Loan Bank, dividend rate percentage				7.67%bmrc_FederalHomeLoanBankDividendRatePercentage / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember
Other assets
Schedule of Cost-method Investments [Line Items]
Federal Home Loan Bank stock	$ 8,200,000us-gaap_FederalHomeLoanBankStock / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember		$ 8,200,000us-gaap_FederalHomeLoanBankStock / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember

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Amount of investment in qualified affordable housing projects.

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