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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 |  | $ 0 us-gaap_LineOfCredit / us-gaap_CreditFacilityAxis = us-gaap_LineOfCreditMember / us-gaap_LineOfCreditFacilityAxis = us-gaap_FederalFundsPurchasedMember | $ 0 us-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 |  | 446,700,000 us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity / us-gaap_CreditFacilityAxis = us-gaap_LineOfCreditMember / us-gaap_LineOfCreditFacilityAxis = us-gaap_FederalHomeLoanBankBorrowingsMember | 450,600,000 us-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 |  | 15,000,000 us-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 |  | 431,500,000 us-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 |  | 241,000 us-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 |  | 0 us-gaap_LineOfCredit / us-gaap_CreditFacilityAxis = us-gaap_LineOfCreditMember / us-gaap_LineOfCreditFacilityAxis = bmrc_FederalHomeLoanBankOvernightBorrowingsMember | 0 us-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 |  | 30,900,000 us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity / us-gaap_CreditFacilityAxis = us-gaap_LineOfCreditMember / us-gaap_LineOfCreditFacilityAxis = us-gaap_FederalReserveBankAdvancesMember | 27,700,000 us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity / us-gaap_CreditFacilityAxis = us-gaap_LineOfCreditMember / us-gaap_LineOfCreditFacilityAxis = us-gaap_FederalReserveBankAdvancesMember
Amount of borrowings outstanding |  | 0 us-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 |  | $ 72,000,000 us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity / us-gaap_CreditFacilityAxis = us-gaap_LineOfCreditMember / us-gaap_LineOfCreditFacilityAxis = us-gaap_FederalFundsPurchasedMember / us-gaap_LongtermDebtTypeAxis = us-gaap_UnsecuredDebtMember | $ 72,000,000 us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity / us-gaap_CreditFacilityAxis = us-gaap_LineOfCreditMember / us-gaap_LineOfCreditFacilityAxis = us-gaap_FederalFundsPurchasedMember / us-gaap_LongtermDebtTypeAxis = us-gaap_UnsecuredDebtMember

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- Definition Debt Instrument, Payment Terms, Periodic Term Lender May Accelerate Due Date + References No definition available. + Details
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- Definition Debt Instrument, Payment Terms, Period Interest-only Payments Due + References No definition available. + Details
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- Definition Long-term Debt, Contractual Term, Number of Years + References No definition available. + Details
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- Definition Contractual interest rate for funds borrowed, under the debt agreement. + 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 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Including both current and noncurrent portions, Federal Home Loan Bank borrowings initially due beyond one year or beyond the normal operating cycle if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 470 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6479118&loc=d3e64650-112822 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 470 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6479336&loc=d3e64702-112823 + Details
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- Definition The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 20, 22 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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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 Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding). + 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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