Investment Securities - Pledged and Transferred Securities (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Financial Instruments Owned and Pledged as Collateral [Line Items]
Securities transferred from available-for-sale to held-to-maturity	$ 0	$ 128,965	$ 129,000
Unrealized pre-tax loss from transfer of available-for-sale securities to held-to-maturity			3,000
Amortization of net unrealized loss on available-for-sale securities transferred to held-to-maturity	136	$ 41	41
Public Deposits
Financial Instruments Owned and Pledged as Collateral [Line Items]
Available-for-sale securities pledged as collateral	102,738		107,829
Trust Deposits
Financial Instruments Owned and Pledged as Collateral [Line Items]
Available-for-sale securities pledged as collateral	754		761
State of California
Financial Instruments Owned and Pledged as Collateral [Line Items]
Available-for-sale securities pledged as collateral	103,492		108,590
Internal checking account
Financial Instruments Owned and Pledged as Collateral [Line Items]
Available-for-sale securities pledged as collateral	$ 2,018		$ 2,026

Amortization Of Net Unrealized Loss On Available-For-Sale Securities Transferred To Held-To-Maturity

No definition available.

Name:	bmrc_AmortizationOfNetUnrealizedLossOnAvailableForSaleSecuritiesTransferredToHeldToMaturity
Namespace Prefix:	bmrc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Available-for-sale Securities, Transferred to Held-to-maturity Securities

+ References

No definition available.

+ Details

Name:	bmrc_AvailableforsaleSecuritiesTransferredtoHeldtomaturitySecurities
Namespace Prefix:	bmrc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Available-for-sale Securities, Transferred to Held-to-maturity Securities, Unrealized Gain (Loss)

+ References

No definition available.

+ Details

Name:	bmrc_AvailableforsaleSecuritiesTransferredtoHeldtomaturitySecuritiesUnrealizedGainLoss
Namespace Prefix:	bmrc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- 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_FinancialInstrumentsOwnedAndPledgedAsCollateralLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- 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=64858031&loc=SL6224234-111729

+ Details

Name:	us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_InvestmentTypeAxis=bmrc_PublicDepositsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=bmrc_TrustDepositsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=bmrc_PledgedToStateOfCaliforniaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=bmrc_CheckingAccountWealthManagementAndTrustServicesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: