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Loans and Allowance for Loan Losses - Pledged Loans (Details) (USD $) In Millions, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014
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Financial Instruments Owned and Pledged as Collateral [Line Items] |  |
Residential loans pledged for FRB borrowings | $ 863.6 us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedLoansReceivablePledgedForFederalHomeLoanBankDebt | $ 868.1 us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedLoansReceivablePledgedForFederalHomeLoanBankDebt
Other residential |  |
Financial Instruments Owned and Pledged as Collateral [Line Items] |  |
Pledged residential loan portfolio to secure borrowing with FRB | 30.9 us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedLoansReceivablePledgedAsCollateral / us-gaap_FinancialInstrumentAxis = bmrc_OtherResidentialFinancingReceivableMember | 27.7 us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedLoansReceivablePledgedAsCollateral / us-gaap_FinancialInstrumentAxis = bmrc_OtherResidentialFinancingReceivableMember
Line of credit | Federal Home Loan Bank Borrowings |  |
Financial Instruments Owned and Pledged as Collateral [Line Items] |  |
Lines of credit | $ 446.7 us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity / us-gaap_CreditFacilityAxis = us-gaap_LineOfCreditMember / us-gaap_LineOfCreditFacilityAxis = us-gaap_FederalHomeLoanBankBorrowingsMember | $ 450.6 us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity / us-gaap_CreditFacilityAxis = us-gaap_LineOfCreditMember / us-gaap_LineOfCreditFacilityAxis = us-gaap_FederalHomeLoanBankBorrowingsMember

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- Definition Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition The sum of the carrying amounts, as of the date of the latest financial statement presented, of loans receivable which are owned but transferred to serve as collateral for the payment of related debt obligations, primarily secured borrowings and repurchase agreements, and for which the transferees are not permitted to sell or re-pledge them to an unrelated party. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 30 -Section 50 -Paragraph 1A -Subparagraph (b)(1)(i) -URI http://asc.fasb.org/extlink&oid=55761124&loc=SL6224234-111729 + Details
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- Definition The carrying amount, as of the date of the latest financial statement presented, of loans receivable which are owned but transferred to serve as collateral for the payment of FHLB debt obligations, and for which the transferee is not permitted to sell or re-pledge them to an unrelated party. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 30 -Section 50 -Paragraph 1A -Subparagraph (b)(1)(i) -URI http://asc.fasb.org/extlink&oid=55761124&loc=SL6224234-111729 + Details
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