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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	$ 1,559us-gaap_ProceedsFromSaleOfAvailableForSaleSecurities	$ 2,023us-gaap_ProceedsFromSaleOfAvailableForSaleSecurities
Gain on sale of held-to-maturity securities	8us-gaap_HeldToMaturitySecuritiesSoldSecurityRealizedGainLoss	7us-gaap_HeldToMaturitySecuritiesSoldSecurityRealizedGainLoss
Proceeds from sale of held-to-maturity securities	0us-gaap_ProceedsFromSaleOfHeldToMaturitySecurities	725us-gaap_ProceedsFromSaleOfHeldToMaturitySecurities
Loss on sale of available-for-sale securities sold		15us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses
State of California
Financial Instruments Owned and Pledged as Collateral [Line Items]
Available-for-sale securities pledged as collateral	69,200us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged / bmrc_FinancialInstrumentsOwnedAndPledgedAsCollateralByPurposeAxis = bmrc_PledgedToStateOfCaliforniaMember		74,700us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged / bmrc_FinancialInstrumentsOwnedAndPledgedAsCollateralByPurposeAxis = bmrc_PledgedToStateOfCaliforniaMember
Public Deposits
Financial Instruments Owned and Pledged as Collateral [Line Items]
Available-for-sale securities pledged as collateral	68,400us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged / bmrc_FinancialInstrumentsOwnedAndPledgedAsCollateralByPurposeAxis = bmrc_PublicDepositsMember		73,800us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged / bmrc_FinancialInstrumentsOwnedAndPledgedAsCollateralByPurposeAxis = bmrc_PublicDepositsMember
Trust Deposits
Financial Instruments Owned and Pledged as Collateral [Line Items]
Available-for-sale securities pledged as collateral	852us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged / bmrc_FinancialInstrumentsOwnedAndPledgedAsCollateralByPurposeAxis = bmrc_TrustDepositsMember		856us-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	$ 1,100us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged / bmrc_FinancialInstrumentsOwnedAndPledgedAsCollateralByPurposeAxis = bmrc_CheckingAccountWealthManagementAndTrustServicesMember		$ 1,100us-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	2bmrc_NumberOfSecuritiesSoldDuringPeriod / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember	1bmrc_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		3bmrc_NumberOfSecuritiesSoldDuringPeriod / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_HeldtomaturitySecuritiesMember

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- Definition

Number of Securities Sold During Period

+ References

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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

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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

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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

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X

- 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

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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

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