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Fair Value of Assets and Liabilities - Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified | 0 Months Ended | 3 Months Ended |
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Nov. 29, 2013 | Mar. 31, 2015 | Dec. 31, 2014
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] |  |  |
Investment securities held to maturity |  | 109,785 us-gaap_HeldToMaturitySecuritiesFairValue | $ 118,643 us-gaap_HeldToMaturitySecuritiesFairValue
Subordinated Debt [Abstract] |  |  |
Number of subordinated debentures acquired | 2 bmrc_BusinessCombinationsNumberofDebtInstrumentsAcquired |  |
Fair value of loan commitments and standby letters of credit |  | 811 us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability | 1,000 us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability
Subordinated debenture |  |  |
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract] |  |  |
Subordinated debentures | 4,950 us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisSubordinatedDebtObligations / us-gaap_LongtermDebtTypeAxis = us-gaap_SubordinatedDebtMember |  |
Subordinated Debt [Abstract] |  |  |
Number of subordinated debentures acquired | 2 bmrc_BusinessCombinationsNumberofDebtInstrumentsAcquired / us-gaap_LongtermDebtTypeAxis = us-gaap_SubordinatedDebtMember |  |
NorCal Community Bancorp Trust I | Subordinated debenture |  |  |
Subordinated Debt [Abstract] |  |  |
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 |
NorCal Community Bancorp Trust II | Subordinated debenture |  |  |
Subordinated Debt [Abstract] |  |  |
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 |
Carrying Value | Fair Value Hierarchy, Level 1 |  |  |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] |  |  |
Cash and cash equivalents |  | 103,164 us-gaap_CashAndCashEquivalentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember | 41,367 us-gaap_CashAndCashEquivalentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember
Carrying Value | Fair Value Hierarchy, Level 2 |  |  |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] |  |  |
Investment securities held to maturity |  | 107,476 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember | 116,437 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember
Interest receivable |  | 5,453 bmrc_InterestReceivableFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember | 5,909 bmrc_InterestReceivableFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract] |  |  |
Deposits |  | 1,585,120 us-gaap_DepositsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember | 1,551,619 us-gaap_DepositsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember
Federal Home Loan Bank borrowings |  | 15,000 us-gaap_FederalHomeLoanBankBorrowingsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember | 15,000 us-gaap_FederalHomeLoanBankBorrowingsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember
Interest payable |  | 208 bmrc_InterestPayableFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember | 213 bmrc_InterestPayableFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember
Carrying Value | Fair Value Hierarchy, Level 3 |  |  |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] |  |  |
Loans, net |  | 1,331,328 us-gaap_LoansReceivableFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember | 1,348,252 us-gaap_LoansReceivableFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract] |  |  |
Subordinated debentures |  | 5,238 us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisSubordinatedDebtObligations / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember | 5,185 us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisSubordinatedDebtObligations / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember
Fair Value | Fair Value Hierarchy, Level 1 |  |  |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] |  |  |
Cash and cash equivalents |  | 103,164 us-gaap_CashAndCashEquivalentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember | 41,367 us-gaap_CashAndCashEquivalentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember
Fair Value | Fair Value Hierarchy, Level 2 |  |  |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] |  |  |
Investment securities held to maturity |  | 109,785 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember | 118,643 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember
Interest receivable |  | 5,453 bmrc_InterestReceivableFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember | 5,909 bmrc_InterestReceivableFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract] |  |  |
Deposits |  | 1,585,907 us-gaap_DepositsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember | 1,552,446 us-gaap_DepositsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember
Federal Home Loan Bank borrowings |  | 15,514 us-gaap_FederalHomeLoanBankBorrowingsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember | 15,484 us-gaap_FederalHomeLoanBankBorrowingsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember
Interest payable |  | 208 bmrc_InterestPayableFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember | 213 bmrc_InterestPayableFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember
Fair Value | Fair Value Hierarchy, Level 3 |  |  |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] |  |  |
Loans, net |  | 1,341,118 us-gaap_LoansReceivableFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember | 1,361,244 us-gaap_LoansReceivableFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract] |  |  |
Subordinated debentures |  | 5,274 us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisSubordinatedDebtObligations / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember | $ 5,290 us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisSubordinatedDebtObligations / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember

X
- Definition Business Combinations, Number of Debt Instruments Acquired + References No definition available. + Details
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- Definition This element represents the disclosures for fair value estimates for financial instruments required in the footnote disclosures to the financial statements. Carrying amount reprsents accrued interest payable on all forms of debt, including deposit liabilities, that has been incurred and is unpaid for time value of money. The carrying value may approximate fair value due to the short-term nature of their settlement dates. + References No definition available. + Details
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- Definition This element represents the disclosures for fair value estimates for financial instruments required in the footnote disclosures to the financial statements. Carrying amount reprsents interest earned but not received from loan borrowers, investment security issuers and on other forms of short-term investment for time value of money. The carrying value may approximate fair value due to the short-term nature of their settlement dates. + References No definition available. + Details
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- Definition Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Cash -URI http://asc.fasb.org/extlink&oid=6506951 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Cash Equivalents -URI http://asc.fasb.org/extlink&oid=6507016 + Details
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Balance Type: | debit
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- Definition Percentage points added to the reference rate to compute the variable rate on the debt instrument. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Fair value portion of deposit liabilities held by the entity, including, but not limited to, foreign and domestic, interest and noninterest bearing, demand deposits, saving deposits, negotiable orders of withdrawal (NOW) and time deposits. + References No definition available. + Details
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Balance Type: | credit
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- Definition The fair value of financial liabilities, which are not recognized in the financial statements (off-balance sheet) because they fail to meet some other criterion for recognition. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=49121117&loc=d3e13433-108611 + 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 Fair value portion of borrowing from Federal Home Loan Bank (FHLBank). + 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 210.9-03.13(3)(a)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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- Details
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- Definition Fair value of investments in debt securities classified as held-to-maturity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 + Details
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- Definition Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Glossary Loan -URI http://asc.fasb.org/extlink&oid=6517073 + Details
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- Details
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- Details
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- Details
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- Details
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