Investment Securities - Securities Sold and Pledged as Collateral (Details) (USD $) In Thousands, unless otherwise specified
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9 Months Ended |
12 Months Ended |
Sep. 30, 2014
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Dec. 31, 2014
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Dec. 31, 2013
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Dec. 31, 2012
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Financial Instruments Owned and Pledged as Collateral [Line Items] |
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Proceeds from sale of securities available for sale |
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$ 2,436us-gaap_ProceedsFromSaleOfAvailableForSaleSecurities |
$ 7,973us-gaap_ProceedsFromSaleOfAvailableForSaleSecurities |
$ 2,186us-gaap_ProceedsFromSaleOfAvailableForSaleSecurities |
Proceeds from sale of securities held to maturity |
2,146us-gaap_ProceedsFromSaleOfHeldToMaturitySecurities |
2,146us-gaap_ProceedsFromSaleOfHeldToMaturitySecurities |
6,442us-gaap_ProceedsFromSaleOfHeldToMaturitySecurities |
0us-gaap_ProceedsFromSaleOfHeldToMaturitySecurities |
Net loss on sale of available for sale securities sold |
11us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses |
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18us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses |
34us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses |
Net gain on sale of held to maturity securities sold |
104us-gaap_HeldToMaturitySecuritiesSoldSecurityRealizedGainLoss |
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17us-gaap_HeldToMaturitySecuritiesSoldSecurityRealizedGainLoss |
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State of California |
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Financial Instruments Owned and Pledged as Collateral [Line Items] |
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Available-for-sale securities pledged as collateral |
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74,700us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged / bmrc_FinancialInstrumentsOwnedAndPledgedAsCollateralByPurposeAxis = bmrc_PledgedToStateOfCaliforniaMember |
61,800us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged / bmrc_FinancialInstrumentsOwnedAndPledgedAsCollateralByPurposeAxis = bmrc_PledgedToStateOfCaliforniaMember |
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Public Deposits |
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Financial Instruments Owned and Pledged as Collateral [Line Items] |
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Available-for-sale securities pledged as collateral |
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73,800us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged / bmrc_FinancialInstrumentsOwnedAndPledgedAsCollateralByPurposeAxis = bmrc_PublicDepositsMember |
61,100us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged / bmrc_FinancialInstrumentsOwnedAndPledgedAsCollateralByPurposeAxis = bmrc_PublicDepositsMember |
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Trust Deposits |
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Financial Instruments Owned and Pledged as Collateral [Line Items] |
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Available-for-sale securities pledged as collateral |
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856us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged / bmrc_FinancialInstrumentsOwnedAndPledgedAsCollateralByPurposeAxis = bmrc_TrustDepositsMember |
732us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged / bmrc_FinancialInstrumentsOwnedAndPledgedAsCollateralByPurposeAxis = bmrc_TrustDepositsMember |
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Internal checking account |
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Financial Instruments Owned and Pledged as Collateral [Line Items] |
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Available-for-sale securities pledged as collateral |
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$ 1,100us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged / bmrc_FinancialInstrumentsOwnedAndPledgedAsCollateralByPurposeAxis = bmrc_CheckingAccountWealthManagementAndTrustServicesMember |
$ 1,100us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged / bmrc_FinancialInstrumentsOwnedAndPledgedAsCollateralByPurposeAxis = bmrc_CheckingAccountWealthManagementAndTrustServicesMember |
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Available-for-sale Securities [Member] |
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Financial Instruments Owned and Pledged as Collateral [Line Items] |
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Number of securities sold during period |
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2bmrc_NumberOfSecuritiesSoldDuringPeriod / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember |
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1bmrc_NumberOfSecuritiesSoldDuringPeriod / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember |
Held-to-maturity Securities [Member] |
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Financial Instruments Owned and Pledged as Collateral [Line Items] |
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Number of securities sold during period |
6bmrc_NumberOfSecuritiesSoldDuringPeriod / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_HeldtomaturitySecuritiesMember |
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