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Borrowings - Subordinated Debt (Details) (USD $)
0 Months Ended 3 Months Ended
Nov. 29, 2013
Mar. 31, 2015
Mar. 31, 2014
Nov. 29, 2013
debenture
Debt Instrument [Line Items]        
Number of subordinated debentures acquired 2bmrc_BusinessCombinationsNumberofDebtInstrumentsAcquired     2bmrc_BusinessCombinationsNumberofDebtInstrumentsAcquired
Accretion of discount on subordinated debentures   $ 53,000us-gaap_AmortizationOfDebtDiscountPremium $ 54,000us-gaap_AmortizationOfDebtDiscountPremium  
Amount guaranteed, on subordinated basis, distributions and other payments on trust preferred securities   8,000,000us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust    
Subordinated debenture        
Debt Instrument [Line Items]        
Number of subordinated debentures acquired 2bmrc_BusinessCombinationsNumberofDebtInstrumentsAcquired
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SubordinatedDebtMember
    2bmrc_BusinessCombinationsNumberofDebtInstrumentsAcquired
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SubordinatedDebtMember
Subordinated debentures 4,950,000us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisSubordinatedDebtObligations
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SubordinatedDebtMember
    4,950,000us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisSubordinatedDebtObligations
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SubordinatedDebtMember
Contractual value of subordinated debt 8,200,000us-gaap_DebtInstrumentCarryingAmount
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SubordinatedDebtMember
8,248,000us-gaap_DebtInstrumentCarryingAmount
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SubordinatedDebtMember
  8,200,000us-gaap_DebtInstrumentCarryingAmount
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SubordinatedDebtMember
Accretion of discount on subordinated debentures   53,000us-gaap_AmortizationOfDebtDiscountPremium
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SubordinatedDebtMember
54,000us-gaap_AmortizationOfDebtDiscountPremium
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SubordinatedDebtMember
 
Debenture distribution deferral period (up to number of years) 5 years      
Subordinated debenture | NorCal Community Bancorp Trust I        
Debt Instrument [Line Items]        
Contractual value of subordinated debt   4,124,000us-gaap_DebtInstrumentCarryingAmount
/ us-gaap_DebtInstrumentAxis
= bmrc_NorCalCommunityBancorpTrustIMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SubordinatedDebtMember
   
Effective interest rate   3.30%us-gaap_DebtInstrumentInterestRateEffectivePercentage
/ us-gaap_DebtInstrumentAxis
= bmrc_NorCalCommunityBancorpTrustIMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SubordinatedDebtMember
   
Basis spread on subordinated debentures 3.05%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_DebtInstrumentAxis
= bmrc_NorCalCommunityBancorpTrustIMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SubordinatedDebtMember
3.05%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_DebtInstrumentAxis
= bmrc_NorCalCommunityBancorpTrustIMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SubordinatedDebtMember
   
Subordinated debenture | NorCal Community Bancorp Trust II        
Debt Instrument [Line Items]        
Contractual value of subordinated debt   $ 4,124,000us-gaap_DebtInstrumentCarryingAmount
/ us-gaap_DebtInstrumentAxis
= bmrc_NorCalCommunityBancorpTrustIIMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SubordinatedDebtMember
   
Effective interest rate   1.67%us-gaap_DebtInstrumentInterestRateEffectivePercentage
/ us-gaap_DebtInstrumentAxis
= bmrc_NorCalCommunityBancorpTrustIIMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SubordinatedDebtMember
   
Basis spread on subordinated debentures 1.40%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_DebtInstrumentAxis
= bmrc_NorCalCommunityBancorpTrustIIMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SubordinatedDebtMember
1.40%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_DebtInstrumentAxis
= bmrc_NorCalCommunityBancorpTrustIIMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SubordinatedDebtMember