Investment Securities - Amortized Cost and Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities and Held-to-maturity Securities [Line Items]
Held to maturity, Amortized Cost	$ 44,438	$ 69,637
Held-to-maturity, Fair Value	45,097	71,054
Held-to-maturity, Gross Unrealized Gains	820	1,459
Held-to-maturity, Gross Unrealized (Losses)	(161)	(42)
Available for sale, Amortized Cost	378,254	417,410
Available-for-sale, at fair value	372,580	417,787
Available-for-sale, Gross Unrealized Gains	575	1,883
Available-for-sale, Gross Unrealized (Losses)	(6,249)	(1,506)
Total investment securities, Amortized Cost	422,692	487,047
Total investment securities, Fair Value	417,677	488,841
Total investment securities, Gross Unrealized Gains	1,395	3,342
Total investment securities, Gross Unrealized (Losses)	(6,410)	(1,548)
MBS pass-through securities
Schedule of Available-for-sale Securities and Held-to-maturity Securities [Line Items]
Available for sale, Amortized Cost	193,998	138,222
Available-for-sale, at fair value	190,566	138,462
Available-for-sale, Gross Unrealized Gains	145	694
Available-for-sale, Gross Unrealized (Losses)	(3,577)	(454)
CMOs issued by FNMA
Schedule of Available-for-sale Securities and Held-to-maturity Securities [Line Items]
Available for sale, Amortized Cost	13,790	18,266
Available-for-sale, at fair value	13,772	18,219
Available-for-sale, Gross Unrealized Gains	91	97
Available-for-sale, Gross Unrealized (Losses)	(109)	(144)
CMOs issued by FHLMC
Schedule of Available-for-sale Securities and Held-to-maturity Securities [Line Items]
Available for sale, Amortized Cost	43,452	22,889
Available-for-sale, at fair value	42,758	22,932
Available-for-sale, Gross Unrealized Gains	37	82
Available-for-sale, Gross Unrealized (Losses)	(731)	(39)
CMOs issued by GNMA
Schedule of Available-for-sale Securities and Held-to-maturity Securities [Line Items]
Available for sale, Amortized Cost	6,844	10,326
Available-for-sale, at fair value	6,945	10,480
Available-for-sale, Gross Unrealized Gains	102	169
Available-for-sale, Gross Unrealized (Losses)	(1)	(15)
Debentures of government-sponsored agencies
Schedule of Available-for-sale Securities and Held-to-maturity Securities [Line Items]
Available for sale, Amortized Cost	35,486	161,690
Available-for-sale, at fair value	35,403	160,892
Available-for-sale, Gross Unrealized Gains	7	28
Available-for-sale, Gross Unrealized (Losses)	(90)	(826)
Privately-issued CMOs
Schedule of Available-for-sale Securities and Held-to-maturity Securities [Line Items]
Available for sale, Amortized Cost	419	3,960
Available-for-sale, at fair value	419	4,150
Available-for-sale, Gross Unrealized Gains	1	190
Available-for-sale, Gross Unrealized (Losses)	(1)	0
Obligations of state and political subdivisions
Schedule of Available-for-sale Securities and Held-to-maturity Securities [Line Items]
Held to maturity, Amortized Cost	30,856	42,919
Held-to-maturity, Fair Value	31,544	44,146
Held-to-maturity, Gross Unrealized Gains	694	1,246
Held-to-maturity, Gross Unrealized (Losses)	(6)	(19)
Available for sale, Amortized Cost	79,306	57,110
Available-for-sale, at fair value	77,701	57,673
Available-for-sale, Gross Unrealized Gains	135	580
Available-for-sale, Gross Unrealized (Losses)	(1,740)	(17)
Corporate bonds
Schedule of Available-for-sale Securities and Held-to-maturity Securities [Line Items]
Held to maturity, Amortized Cost	3,519	15,072
Held-to-maturity, Fair Value	3,518	15,098
Held-to-maturity, Gross Unrealized Gains	0	42
Held-to-maturity, Gross Unrealized (Losses)	(1)	(16)
Available for sale, Amortized Cost	4,959	4,947
Available-for-sale, at fair value	5,016	4,979
Available-for-sale, Gross Unrealized Gains	57	43
Available-for-sale, Gross Unrealized (Losses)	0	(11)
MBS pass-through securities
Schedule of Available-for-sale Securities and Held-to-maturity Securities [Line Items]
Held to maturity, Amortized Cost	10,063	11,646
Held-to-maturity, Fair Value	10,035	11,810
Held-to-maturity, Gross Unrealized Gains	126	171
Held-to-maturity, Gross Unrealized (Losses)	$ (154)	$ (7)

Available-for-sale Securities and Held-to-maturity Securities, Amortized Cost Basis

No definition available.

Name:	bmrc_AvailableForSaleSecuritiesAndHeldToMaturitySecuritiesAmortizedCostBasis
Namespace Prefix:	bmrc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair Value of Available-for-sale Securities and Held-to-maturity Securities.

+ References

No definition available.

+ Details

Name:	bmrc_AvailableForSaleSecuritiesAndHeldToMaturitySecuritiesFairValue
Namespace Prefix:	bmrc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Available-for-sale Securities and Held-to-maturity Securities, Accumulated Gross Unrealized Gain

+ References

No definition available.

+ Details

Name:	bmrc_AvailableforsaleSecuritiesandHeldtomaturitySecuritiesAccumulatedGrossUnrealizedGain
Namespace Prefix:	bmrc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Available-for-sale Securities and Held-to-maturity Securities, Accumulated Gross Unrealized Loss

+ References

No definition available.

+ Details

Name:	bmrc_AvailableforsaleSecuritiesandHeldtomaturitySecuritiesAccumulatedGrossUnrealizedLoss
Namespace Prefix:	bmrc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Schedule of Available-for-sale Securities and Held-to-maturity Securities [Line Items]

+ References

No definition available.

+ Details

Name:	bmrc_ScheduleOfAvailableForSaleSecuritiesAndHeldToMaturitySecuritiesLineItems
Namespace Prefix:	bmrc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26610-111562

+ Details

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

- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563

+ Details

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

- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563

+ Details

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

- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

Amount of accumulated unrealized gain on investments in debt securities classified as held-to-maturity.

+ References

+ Details

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

- Definition

Amount of accumulated unrealized loss on investments in debt securities classified as held-to-maturity.

+ References

+ Details

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

- Definition

Amount before other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

+ References

No definition available.

+ Details

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

- Definition

Fair value of investments in debt securities classified as held-to-maturity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

+ Details

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

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_AgencySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtSecurityAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtSecurityAxis=us-gaap_CorporateBondSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtSecurityAxis=us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: