Loans and Allowance for Credit Losses on Loans - Schedule of Allowance for Credit Losses on Loans (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2024	Jun. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 30,675		$ 23,832	$ 25,172	$ 22,983
Provision (Reversal)	0	$ 5,200	425	5,550	1,275
(Charge-offs)	0		(6)	(51)	(29)
Recoveries	0		9	4	31
Ending balance	30,675	30,675	24,260	30,675	24,260
Commercial loans \| Commercial and industrial
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	1,876		1,831	1,712	1,794
Provision (Reversal)	(158)		(20)	38	16
(Charge-offs)	0		(4)	(33)	(7)
Recoveries	0		1	1	5
Ending balance	1,718	1,876	1,808	1,718	1,808
Commercial real estate loans \| Commercial owner-occupied
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	2,408		2,589	2,476	2,487
Provision (Reversal)	(63)		(59)	(131)	43
(Charge-offs)	0		0	0	0
Recoveries	0		0	0	0
Ending balance	2,345	2,408	2,530	2,345	2,530
Commercial real estate loans \| Commercial non-owner occupied
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	22,165		13,201	14,933	12,676
Provision (Reversal)	(65)		423	7,167	948
(Charge-offs)	0		0	0	0
Recoveries	0		0	0	0
Ending balance	22,100	22,165	13,624	22,100	13,624
Commercial real estate loans \| Construction
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	874		1,942	1,832	1,937
Provision (Reversal)	(214)		68	(1,172)	56
(Charge-offs)	0		0	0	0
Recoveries	0		8	0	25
Ending balance	660	874	2,018	660	2,018
Residential loans \| Home equity
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	650		561	552	558
Provision (Reversal)	(3)		4	95	7
(Charge-offs)	0		0	0	0
Recoveries	0		0	0	0
Ending balance	647	650	565	647	565
Residential loans \| Other residential
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	769		599	653	595
Provision (Reversal)	376		6	492	10
(Charge-offs)	0		0	0	0
Recoveries	0		0	0	0
Ending balance	1,145	769	605	1,145	605
Consumer loans \| Installment and other consumer loans
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	910		940	976	868
Provision (Reversal)	10		13	(41)	104
(Charge-offs)	0		(2)	(18)	(22)
Recoveries	0		0	3	1
Ending balance	920	910	951	920	951
Unallocated
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	1,023		2,169	2,038	2,068
Provision (Reversal)	117		(10)	(898)	91
(Charge-offs)	0		0	0	0
Recoveries	0		0	0	0
Ending balance	$ 1,140	$ 1,023	$ 2,159	$ 1,140	$ 2,159

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

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

- Definition

Amount excluding accrued interest, of allowance for credit loss on financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479344/326-20-45-1

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterest
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, excluding accrued interest, of increase in allowance for credit loss on financing receivable from recovery. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossRecovery
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount, excluding accrued interest, of decrease in allowance for credit loss on financing receivable from writeoff. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-6

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossWriteoff
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount, excluding accrued interest, of credit loss expense (reversal of expense) on financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestCreditLossExpenseReversal
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=bmrc_CommercialAndIndustrialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=bmrc_CommercialRealEstateLoanOwnerOccupiedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=bmrc_CommercialRealEstateLoanNonOwnerOccupiedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConstructionLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_HomeEquityLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=bmrc_OtherResidentialLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=bmrc_InstallmentAndOtherConsumerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_UnallocatedFinancingReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: