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Borrowings - Subordinated Debt (Details) (USD $) | 0 Months Ended | 3 Months Ended |
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Nov. 29, 2013 | Mar. 31, 2015 | Mar. 31, 2014 | Nov. 29, 2013 debenture
Debt Instrument [Line Items] |  |  |  |
Number of subordinated debentures acquired | 2 bmrc_BusinessCombinationsNumberofDebtInstrumentsAcquired |  |  | 2 bmrc_BusinessCombinationsNumberofDebtInstrumentsAcquired
Accretion of discount on subordinated debentures |  | $ 53,000 us-gaap_AmortizationOfDebtDiscountPremium | $ 54,000 us-gaap_AmortizationOfDebtDiscountPremium |
Amount guaranteed, on subordinated basis, distributions and other payments on trust preferred securities |  | 8,000,000 us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust |  |
Subordinated debenture |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Number of subordinated debentures acquired | 2 bmrc_BusinessCombinationsNumberofDebtInstrumentsAcquired / us-gaap_LongtermDebtTypeAxis = us-gaap_SubordinatedDebtMember |  |  | 2 bmrc_BusinessCombinationsNumberofDebtInstrumentsAcquired / us-gaap_LongtermDebtTypeAxis = us-gaap_SubordinatedDebtMember
Subordinated debentures | 4,950,000 us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisSubordinatedDebtObligations / us-gaap_LongtermDebtTypeAxis = us-gaap_SubordinatedDebtMember |  |  | 4,950,000 us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisSubordinatedDebtObligations / us-gaap_LongtermDebtTypeAxis = us-gaap_SubordinatedDebtMember
Contractual value of subordinated debt | 8,200,000 us-gaap_DebtInstrumentCarryingAmount / us-gaap_LongtermDebtTypeAxis = us-gaap_SubordinatedDebtMember | 8,248,000 us-gaap_DebtInstrumentCarryingAmount / us-gaap_LongtermDebtTypeAxis = us-gaap_SubordinatedDebtMember |  | 8,200,000 us-gaap_DebtInstrumentCarryingAmount / us-gaap_LongtermDebtTypeAxis = us-gaap_SubordinatedDebtMember
Accretion of discount on subordinated debentures |  | 53,000 us-gaap_AmortizationOfDebtDiscountPremium / us-gaap_LongtermDebtTypeAxis = us-gaap_SubordinatedDebtMember | 54,000 us-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 |  | 4,124,000 us-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 |  | $ 4,124,000 us-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 |  |

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- Definition Amount of noncash expense included in interest expense to amortize debt discount and premium associated with the related debt instruments. Excludes amortization of financing costs. Alternate captions include noncash interest expense. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 1A -URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28541-108399 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3602-108585 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.8) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 + Details
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- Definition Amount of long-term debt before deduction of unamortized discount or premium. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt, with initial maturities beyond one year or the normal operating cycle, if longer. + 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) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 20 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 9 + Details
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- Definition Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399 Reference 2: 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 Fair value of subordinated debt obligations including, but not limited to, subordinated loan, subordinated bond, subordinated debenture or junior debt. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a),(b) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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- Definition Including the current and noncurrent portions, the carrying value of junior subordinated long-term debt owed to unconsolidated subsidiary trusts as of the balance sheet date. Such debt places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets and was initially scheduled to be repaid after 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 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 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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