XML 46 R35.htm IDEA: XBRL DOCUMENT
v3.20.2

Investment Securities - Non-Marketable Securities Included in Other Assets (Details) | 9 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Sep. 30, 2020 USD ($) $ / shares shares | Sep. 30, 2019 USD ($) | Dec. 31, 2019 USD ($) shares | Oct. 29, 2020
Schedule of Equity Method Investments [Line Items] |  |  |  |
Federal home loan bank stock, par value (in usd per share) | $ / shares | $ 100 |  |  |
Investments in low income housing tax credit funds | $ 3,700,000 |  | $ 4,100,000 |
Low income housing tax credits and other tax benefits | 490,000 |  |  |
Low income housing amortization expense | 412,000 |  |  |
Unfunded commitments for low income housing tax credit funds | $ 886,000 |  |  |
Subsequent event |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Federal home loan bank, dividend rate percentage |  |  |  | 5.00%
Visa Inc. Class B common stock |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Number of shares of securities carried at cost (in shares) | shares | 10,439 |  | 10,439 |
Carrying value of securities carried at cost | $ 0 |  | $ 0 |
Fair value of Class B common stock | $ 3,400,000 |  | $ 3,200,000 |
Visa Inc. | Visa Inc. Class B common stock |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Conversion rate on common stock | 1.6228 |  | 1.6228 |
Asset Derivatives |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Federal home loan bank stock | $ 11,900,000 |  | $ 11,700,000 |
Low-income housing tax credit investment |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Impairment losses | $ 0 | $ 0 |  |

X
- Definition Affordable Housing Program, Unfunded Commitments + References No definition available. + Details
Name: | bmrc_AffordableHousingProgramUnfundedCommitments
Namespace Prefix: | bmrc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amortization Method Qualified Affordable Housing Project Investments, Tax Credits + References No definition available. + Details
Name: | bmrc_AmortizationMethodQualifiedAffordableHousingProjectInvestmentsTaxCredits
Namespace Prefix: | bmrc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Federal Home Loan Bank, Dividend Rate, Percentage + References No definition available. + Details
Name: | bmrc_FederalHomeLoanBankDividendRatePercentage
Namespace Prefix: | bmrc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Federal Home Loan Bank Stock, Par Value + References No definition available. + Details
Name: | bmrc_FederalHomeLoanBankStockParValue
Namespace Prefix: | bmrc_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | instant

X
- Definition Amount of investment in qualified affordable housing projects. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 740 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=116633289&loc=SL49122993-202985 + Details
Name: | us-gaap_AmortizationMethodQualifiedAffordableHousingProjectInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of amortization expense of investment in qualified affordable housing projects. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 740 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=116633289&loc=SL49122993-202985 + Details
Name: | us-gaap_AmortizationMethodQualifiedAffordableHousingProjectInvestmentsAmortization
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 45 -Paragraph 1A -URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740 + Details
Name: | us-gaap_EquitySecuritiesFvNi
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Cost of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). Excludes equity method investment and investment in equity security without readily determinable fair value. + References No definition available. + Details
Name: | us-gaap_EquitySecuritiesFvNiCost
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Federal Home Loan Bank (FHLB) stock represents an equity interest in a FHLB. It does not have a readily determinable fair value because its ownership is restricted and it lacks a market (liquidity). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 325 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=75031641&loc=d3e63345-112809 + Details
Name: | us-gaap_FederalHomeLoanBankStock
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 8A -URI http://asc.fasb.org/extlink&oid=121553693&loc=SL6284422-111562 + Details
Name: | us-gaap_ImpairmentOfInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Balance held at close of period in number of shares. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 210 -Section 50 -Paragraph 6 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=99383244&loc=d3e12121-115841 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-12B(Column B)) -URI http://asc.fasb.org/extlink&oid=120401592&loc=d3e611197-123010 + Details
Name: | us-gaap_InvestmentOwnedBalanceShares
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_ScheduleOfEquityMethodInvestmentsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Ratio applied to the conversion of stock split, for example but not limited to, one share converted to two or two shares converted to one. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 4 -Subparagraph (SAB TOPIC 4.C) -URI http://asc.fasb.org/extlink&oid=120397183&loc=d3e187143-122770 + Details
Name: | us-gaap_StockholdersEquityNoteStockSplitConversionRatio1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=bmrc_VisaInc.ClassBCommonStockMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=bmrc_VisaIncMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementClassOfStockAxis=us-gaap_CommonClassBMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_BalanceSheetLocationAxis=us-gaap_OtherAssetsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InvestmentTypeAxis=bmrc_LowIncomeHousingTaxCreditInvestmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |