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Investment Securities - Securities Sold and Pledged as Collateral (Details) (USD $)
In Thousands, unless otherwise specified
9 Months Ended 12 Months Ended
Sep. 30, 2014
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Financial Instruments Owned and Pledged as Collateral [Line Items]        
Proceeds from sale of securities available for sale   $ 2,436us-gaap_ProceedsFromSaleOfAvailableForSaleSecurities $ 7,973us-gaap_ProceedsFromSaleOfAvailableForSaleSecurities $ 2,186us-gaap_ProceedsFromSaleOfAvailableForSaleSecurities
Proceeds from sale of securities held to maturity 2,146us-gaap_ProceedsFromSaleOfHeldToMaturitySecurities 2,146us-gaap_ProceedsFromSaleOfHeldToMaturitySecurities 6,442us-gaap_ProceedsFromSaleOfHeldToMaturitySecurities 0us-gaap_ProceedsFromSaleOfHeldToMaturitySecurities
Net loss on sale of available for sale securities sold 11us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses   18us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses 34us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses
Net gain on sale of held to maturity securities sold 104us-gaap_HeldToMaturitySecuritiesSoldSecurityRealizedGainLoss   17us-gaap_HeldToMaturitySecuritiesSoldSecurityRealizedGainLoss  
State of California        
Financial Instruments Owned and Pledged as Collateral [Line Items]        
Available-for-sale securities pledged as collateral   74,700us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged
/ bmrc_FinancialInstrumentsOwnedAndPledgedAsCollateralByPurposeAxis
= bmrc_PledgedToStateOfCaliforniaMember
61,800us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged
/ bmrc_FinancialInstrumentsOwnedAndPledgedAsCollateralByPurposeAxis
= bmrc_PledgedToStateOfCaliforniaMember
 
Public Deposits        
Financial Instruments Owned and Pledged as Collateral [Line Items]        
Available-for-sale securities pledged as collateral   73,800us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged
/ bmrc_FinancialInstrumentsOwnedAndPledgedAsCollateralByPurposeAxis
= bmrc_PublicDepositsMember
61,100us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged
/ bmrc_FinancialInstrumentsOwnedAndPledgedAsCollateralByPurposeAxis
= bmrc_PublicDepositsMember
 
Trust Deposits        
Financial Instruments Owned and Pledged as Collateral [Line Items]        
Available-for-sale securities pledged as collateral   856us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged
/ bmrc_FinancialInstrumentsOwnedAndPledgedAsCollateralByPurposeAxis
= bmrc_TrustDepositsMember
732us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged
/ bmrc_FinancialInstrumentsOwnedAndPledgedAsCollateralByPurposeAxis
= bmrc_TrustDepositsMember
 
Internal checking account        
Financial Instruments Owned and Pledged as Collateral [Line Items]        
Available-for-sale securities pledged as collateral   $ 1,100us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged
/ bmrc_FinancialInstrumentsOwnedAndPledgedAsCollateralByPurposeAxis
= bmrc_CheckingAccountWealthManagementAndTrustServicesMember
$ 1,100us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged
/ bmrc_FinancialInstrumentsOwnedAndPledgedAsCollateralByPurposeAxis
= bmrc_CheckingAccountWealthManagementAndTrustServicesMember
 
Available-for-sale Securities [Member]        
Financial Instruments Owned and Pledged as Collateral [Line Items]        
Number of securities sold during period   2bmrc_NumberOfSecuritiesSoldDuringPeriod
/ us-gaap_GainLossOnInvestmentsByCategoryAxis
= us-gaap_AvailableforsaleSecuritiesMember
  1bmrc_NumberOfSecuritiesSoldDuringPeriod
/ us-gaap_GainLossOnInvestmentsByCategoryAxis
= us-gaap_AvailableforsaleSecuritiesMember
Held-to-maturity Securities [Member]        
Financial Instruments Owned and Pledged as Collateral [Line Items]        
Number of securities sold during period 6bmrc_NumberOfSecuritiesSoldDuringPeriod
/ us-gaap_GainLossOnInvestmentsByCategoryAxis
= us-gaap_HeldtomaturitySecuritiesMember