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Investment Securities - Securities Sold and Pledged as Collateral (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended |
--- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014 | Dec. 31, 2014
Financial Instruments Owned and Pledged as Collateral [Line Items] |  |  |
Proceeds from sale of available-for-sale securities | $ 1,559 us-gaap_ProceedsFromSaleOfAvailableForSaleSecurities | $ 2,023 us-gaap_ProceedsFromSaleOfAvailableForSaleSecurities |
Gain on sale of held-to-maturity securities | 8 us-gaap_HeldToMaturitySecuritiesSoldSecurityRealizedGainLoss | 7 us-gaap_HeldToMaturitySecuritiesSoldSecurityRealizedGainLoss |
Proceeds from sale of held-to-maturity securities | 0 us-gaap_ProceedsFromSaleOfHeldToMaturitySecurities | 725 us-gaap_ProceedsFromSaleOfHeldToMaturitySecurities |
Loss on sale of available-for-sale securities sold |  | 15 us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses |
State of California |  |  |
Financial Instruments Owned and Pledged as Collateral [Line Items] |  |  |
Available-for-sale securities pledged as collateral | 69,200 us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged / bmrc_FinancialInstrumentsOwnedAndPledgedAsCollateralByPurposeAxis = bmrc_PledgedToStateOfCaliforniaMember |  | 74,700 us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged / bmrc_FinancialInstrumentsOwnedAndPledgedAsCollateralByPurposeAxis = bmrc_PledgedToStateOfCaliforniaMember
Public Deposits |  |  |
Financial Instruments Owned and Pledged as Collateral [Line Items] |  |  |
Available-for-sale securities pledged as collateral | 68,400 us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged / bmrc_FinancialInstrumentsOwnedAndPledgedAsCollateralByPurposeAxis = bmrc_PublicDepositsMember |  | 73,800 us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged / bmrc_FinancialInstrumentsOwnedAndPledgedAsCollateralByPurposeAxis = bmrc_PublicDepositsMember
Trust Deposits |  |  |
Financial Instruments Owned and Pledged as Collateral [Line Items] |  |  |
Available-for-sale securities pledged as collateral | 852 us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged / bmrc_FinancialInstrumentsOwnedAndPledgedAsCollateralByPurposeAxis = bmrc_TrustDepositsMember |  | 856 us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged / bmrc_FinancialInstrumentsOwnedAndPledgedAsCollateralByPurposeAxis = bmrc_TrustDepositsMember
Internal checking account |  |  |
Financial Instruments Owned and Pledged as Collateral [Line Items] |  |  |
Available-for-sale securities pledged as collateral | $ 1,100 us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged / bmrc_FinancialInstrumentsOwnedAndPledgedAsCollateralByPurposeAxis = bmrc_CheckingAccountWealthManagementAndTrustServicesMember |  | $ 1,100 us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged / bmrc_FinancialInstrumentsOwnedAndPledgedAsCollateralByPurposeAxis = bmrc_CheckingAccountWealthManagementAndTrustServicesMember
Available-for-sale Securities |  |  |
Financial Instruments Owned and Pledged as Collateral [Line Items] |  |  |
Number of securities sold during period | 2 bmrc_NumberOfSecuritiesSoldDuringPeriod / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember | 1 bmrc_NumberOfSecuritiesSoldDuringPeriod / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember |
Held-to-maturity Securities |  |  |
Financial Instruments Owned and Pledged as Collateral [Line Items] |  |  |
Number of securities sold during period |  | 3 bmrc_NumberOfSecuritiesSoldDuringPeriod / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_HeldtomaturitySecuritiesMember |

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- Definition Number of Securities Sold During Period + References No definition available. + Details
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- Definition This item represents the gross loss realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563 + Details
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- Definition This item represents the gain (loss) realized on the sale of held-to-maturity securities disposed of during the period. Additionally, this item may include, if not separately presented, any losses recognized for other than temporary impairments (OTTI) of the subject investments in debt securities which are being or have been recognized in earnings during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27405-111563 + Details
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- Definition Sum of the carrying amounts as of the date of the latest financial statement presented of securities which are owned but transferred to another party to serve as collateral to partially or fully secure a debt obligation, repurchase agreement or other current or potential obligation, 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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- Definition The cash inflow associated with the sale of debt and equity securities classified as available-for-sale securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Investing Activities -URI http://asc.fasb.org/extlink&oid=6516133 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=49167202&loc=d3e26853-111562 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -Subparagraph (a),(b) -URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3179-108585 + Details
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- Definition The cash inflow associated with the sale of securities that had been designated as held-to-maturity. Excludes proceeds from maturities, prepayments and calls by the issuer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Investing Activities -URI http://asc.fasb.org/extlink&oid=6516133 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=49167202&loc=d3e26853-111562 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3179-108585 + Details
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