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Borrowings - Lines of Credit (Details) (USD $)
0 Months Ended 3 Months Ended
Feb. 05, 2008
Mar. 31, 2015
Dec. 31, 2014
Line of Credit Facility [Line Items]      
Periodic term lender may accelerate due date   3 months  
Federal Funds Purchased | Line of credit      
Line of Credit Facility [Line Items]      
Amount of borrowings outstanding   $ 0us-gaap_LineOfCredit
/ us-gaap_CreditFacilityAxis
= us-gaap_LineOfCreditMember
/ us-gaap_LineOfCreditFacilityAxis
= us-gaap_FederalFundsPurchasedMember
$ 0us-gaap_LineOfCredit
/ us-gaap_CreditFacilityAxis
= us-gaap_LineOfCreditMember
/ us-gaap_LineOfCreditFacilityAxis
= us-gaap_FederalFundsPurchasedMember
Federal Home Loan Bank Borrowings | Line of credit      
Line of Credit Facility [Line Items]      
Lines of credit   446,700,000us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
/ us-gaap_CreditFacilityAxis
= us-gaap_LineOfCreditMember
/ us-gaap_LineOfCreditFacilityAxis
= us-gaap_FederalHomeLoanBankBorrowingsMember
450,600,000us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
/ us-gaap_CreditFacilityAxis
= us-gaap_LineOfCreditMember
/ us-gaap_LineOfCreditFacilityAxis
= us-gaap_FederalHomeLoanBankBorrowingsMember
Term of FHLB borrowing agreements 10 years    
Principal amount of borrowing agreements with FHLB   15,000,000us-gaap_FederalHomeLoanBankAdvancesLongTerm
/ us-gaap_CreditFacilityAxis
= us-gaap_LineOfCreditMember
/ us-gaap_LineOfCreditFacilityAxis
= us-gaap_FederalHomeLoanBankBorrowingsMember
 
Fixed interest rate of FHLB borrowing agreement 2.07%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_CreditFacilityAxis
= us-gaap_LineOfCreditMember
/ us-gaap_LineOfCreditFacilityAxis
= us-gaap_FederalHomeLoanBankBorrowingsMember
   
Periodic term interest-only payments are due   3 months  
Remaining available line of credit from FHLB   431,500,000us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
/ us-gaap_CreditFacilityAxis
= us-gaap_LineOfCreditMember
/ us-gaap_LineOfCreditFacilityAxis
= us-gaap_FederalHomeLoanBankBorrowingsMember
 
Federal Home Loan Bank Borrowings | Letters of credit      
Line of Credit Facility [Line Items]      
Amount of borrowings outstanding   241,000us-gaap_LineOfCredit
/ us-gaap_CreditFacilityAxis
= us-gaap_LetterOfCreditMember
/ us-gaap_LineOfCreditFacilityAxis
= us-gaap_FederalHomeLoanBankBorrowingsMember
 
Federal Home Loan Bank Overnight Borrowings | Line of credit      
Line of Credit Facility [Line Items]      
Amount of borrowings outstanding   0us-gaap_LineOfCredit
/ us-gaap_CreditFacilityAxis
= us-gaap_LineOfCreditMember
/ us-gaap_LineOfCreditFacilityAxis
= bmrc_FederalHomeLoanBankOvernightBorrowingsMember
0us-gaap_LineOfCredit
/ us-gaap_CreditFacilityAxis
= us-gaap_LineOfCreditMember
/ us-gaap_LineOfCreditFacilityAxis
= bmrc_FederalHomeLoanBankOvernightBorrowingsMember
Federal Reserve Line of Credit | Line of credit      
Line of Credit Facility [Line Items]      
Lines of credit   30,900,000us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
/ us-gaap_CreditFacilityAxis
= us-gaap_LineOfCreditMember
/ us-gaap_LineOfCreditFacilityAxis
= us-gaap_FederalReserveBankAdvancesMember
27,700,000us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
/ us-gaap_CreditFacilityAxis
= us-gaap_LineOfCreditMember
/ us-gaap_LineOfCreditFacilityAxis
= us-gaap_FederalReserveBankAdvancesMember
Amount of borrowings outstanding   0us-gaap_LineOfCredit
/ us-gaap_CreditFacilityAxis
= us-gaap_LineOfCreditMember
/ us-gaap_LineOfCreditFacilityAxis
= us-gaap_FederalReserveBankAdvancesMember
 
Unsecured Debt | Federal Funds Purchased | Line of credit      
Line of Credit Facility [Line Items]      
Lines of credit   $ 72,000,000us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
/ us-gaap_CreditFacilityAxis
= us-gaap_LineOfCreditMember
/ us-gaap_LineOfCreditFacilityAxis
= us-gaap_FederalFundsPurchasedMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_UnsecuredDebtMember
$ 72,000,000us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
/ us-gaap_CreditFacilityAxis
= us-gaap_LineOfCreditMember
/ us-gaap_LineOfCreditFacilityAxis
= us-gaap_FederalFundsPurchasedMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_UnsecuredDebtMember