XML 47 R37.htm IDEA: XBRL DOCUMENT
v3.24.3

Investment Securities - Schedule of Unrealized Loss on Investments (Details) - USD ($) $ in Thousands | Sep. 30, 2024 | Dec. 31, 2023
--- | --- | ---
Debt Securities, Held-to-maturity, Fair Value |  |
Less than 12 continuous months | $ 20,764 | $ 62,124
Greater than or equal to 12 continuous months | 757,652 | 738,690
Total securities in a loss position | 778,416 | 800,814
Debt Securities, Held-to-maturity, Unrealized Loss |  |
Less than 12 continuous months | (1,240) | (676)
Greater than or equal to 12 continuous months | (88,167) | (109,767)
Held-to-maturity, gross unrealized losses | (89,407) | (110,443)
Debt Securities, Available-For-Sale, Fair Value |  |
Available-for-sale, less than 12 continuous months, fair value | 76,278 | 1,901
Available-for-sale, greater than 12 continuous months, fair value | 234,867 | 549,289
Available-for-sale, total securities in a loss position, fair value | 311,145 | 551,190
Debt Securities, Available-for-sale, Unrealized Loss |  |
Available-for-sale, less than 12 continuous months, unrealized loss | (92) | (8)
Available-for-sale, greater than 12 continuous months, unrealized loss | (25,213) | (61,446)
Available-for-sale, total securities in a loss position, unrealized loss | (25,305) | (61,454)
Marketable Securities, Fair Value |  |
Marketable securities, less than 12 continuous months, fair value | 97,042 | 64,025
Marketable securities, greater than 12 continuous months, fair value | 992,519 | 1,287,979
Marketable securities, total securities in a loss position, fair value | 1,089,561 | 1,352,004
Marketable Securities, Continuous Unrealized Loss |  |
Marketable securities, less than 12 continuous months, unrealized loss | (1,332) | (684)
Marketable securities, greater than 12 continuous months, unrealized loss | (113,380) | (171,213)
Marketable securities, total securities in a loss position, unrealized loss | (114,712) | (171,897)
CMBS issued by FHLMC, FNMA and GNMA |  |
Debt Securities, Held-to-maturity, Fair Value |  |
Less than 12 continuous months | 8,888 | 10,988
Greater than or equal to 12 continuous months | 206,503 | 201,383
Total securities in a loss position | 215,391 | 212,371
Debt Securities, Held-to-maturity, Unrealized Loss |  |
Less than 12 continuous months | (35) | (244)
Greater than or equal to 12 continuous months | (28,196) | (34,826)
Held-to-maturity, gross unrealized losses | (28,231) | (35,070)
Debt Securities, Available-For-Sale, Fair Value |  |
Available-for-sale, less than 12 continuous months, fair value | 76,278 | 1,235
Available-for-sale, greater than 12 continuous months, fair value | 60,550 | 146,454
Available-for-sale, total securities in a loss position, fair value | 136,828 | 147,689
Debt Securities, Available-for-sale, Unrealized Loss |  |
Available-for-sale, less than 12 continuous months, unrealized loss | (92) | (7)
Available-for-sale, greater than 12 continuous months, unrealized loss | (3,850) | (13,272)
Available-for-sale, total securities in a loss position, unrealized loss | (3,942) | (13,279)
CMOs issued by FHLMC, FNMA and GNMA |  |
Debt Securities, Held-to-maturity, Fair Value |  |
Less than 12 continuous months | 7,819 | 51,136
Greater than or equal to 12 continuous months | 182,278 | 156,515
Total securities in a loss position | 190,097 | 207,651
Debt Securities, Held-to-maturity, Unrealized Loss |  |
Less than 12 continuous months | (1,204) | (432)
Greater than or equal to 12 continuous months | (11,528) | (16,451)
Held-to-maturity, gross unrealized losses | (12,732) | (16,883)
Debt Securities, Available-For-Sale, Fair Value |  |
Available-for-sale, less than 12 continuous months, fair value | 0 | 0
Available-for-sale, greater than 12 continuous months, fair value | 36,052 | 136,269
Available-for-sale, total securities in a loss position, fair value | 36,052 | 136,269
Debt Securities, Available-for-sale, Unrealized Loss |  |
Available-for-sale, less than 12 continuous months, unrealized loss | 0 | 0
Available-for-sale, greater than 12 continuous months, unrealized loss | (5,248) | (17,420)
Available-for-sale, total securities in a loss position, unrealized loss | (5,248) | (17,420)
MBS pass-through securities issued by FHLMC, FNMA and GNMA |  |
Debt Securities, Held-to-maturity, Fair Value |  |
Less than 12 continuous months | 0 | 0
Greater than or equal to 12 continuous months | 169,133 | 181,656
Total securities in a loss position | 169,133 | 181,656
Debt Securities, Held-to-maturity, Unrealized Loss |  |
Less than 12 continuous months | 0 | 0
Greater than or equal to 12 continuous months | (23,301) | (27,326)
Held-to-maturity, gross unrealized losses | (23,301) | (27,326)
Debt Securities, Available-For-Sale, Fair Value |  |
Available-for-sale, less than 12 continuous months, fair value | 0 | 0
Available-for-sale, greater than 12 continuous months, fair value | 27,535 | 68,237
Available-for-sale, total securities in a loss position, fair value | 27,535 | 68,237
Debt Securities, Available-for-sale, Unrealized Loss |  |
Available-for-sale, less than 12 continuous months, unrealized loss | 0 | 0
Available-for-sale, greater than 12 continuous months, unrealized loss | (3,823) | (9,168)
Available-for-sale, total securities in a loss position, unrealized loss | (3,823) | (9,168)
SBA-backed securities |  |
Debt Securities, Held-to-maturity, Fair Value |  |
Less than 12 continuous months | 0 | 0
Greater than or equal to 12 continuous months | 1,471 | 1,763
Total securities in a loss position | 1,471 | 1,763
Debt Securities, Held-to-maturity, Unrealized Loss |  |
Less than 12 continuous months | 0 | 0
Greater than or equal to 12 continuous months | (43) | (90)
Held-to-maturity, gross unrealized losses | (43) | (90)
Debt Securities, Available-For-Sale, Fair Value |  |
Available-for-sale, less than 12 continuous months, fair value | 0 | 0
Available-for-sale, greater than 12 continuous months, fair value | 314 | 19,471
Available-for-sale, total securities in a loss position, fair value | 314 | 19,471
Debt Securities, Available-for-sale, Unrealized Loss |  |
Available-for-sale, less than 12 continuous months, unrealized loss | 0 | 0
Available-for-sale, greater than 12 continuous months, unrealized loss | (18) | (1,655)
Available-for-sale, total securities in a loss position, unrealized loss | (18) | (1,655)
Debentures of government- sponsored agencies |  |
Debt Securities, Held-to-maturity, Fair Value |  |
Less than 12 continuous months | 0 | 0
Greater than or equal to 12 continuous months | 123,582 | 124,132
Total securities in a loss position | 123,582 | 124,132
Debt Securities, Held-to-maturity, Unrealized Loss |  |
Less than 12 continuous months | 0 | 0
Greater than or equal to 12 continuous months | (17,775) | (21,994)
Held-to-maturity, gross unrealized losses | (17,775) | (21,994)
Debt Securities, Available-For-Sale, Fair Value |  |
Available-for-sale, less than 12 continuous months, fair value | 0 | 0
Available-for-sale, greater than 12 continuous months, fair value | 7,508 | 66,862
Available-for-sale, total securities in a loss position, fair value | 7,508 | 66,862
Debt Securities, Available-for-sale, Unrealized Loss |  |
Available-for-sale, less than 12 continuous months, unrealized loss | 0 | 0
Available-for-sale, greater than 12 continuous months, unrealized loss | (1,463) | (7,037)
Available-for-sale, total securities in a loss position, unrealized loss | (1,463) | (7,037)
U.S. Treasury securities |  |
Debt Securities, Available-For-Sale, Fair Value |  |
Available-for-sale, less than 12 continuous months, fair value | 0 | 0
Available-for-sale, greater than 12 continuous months, fair value | 10,975 | 10,623
Available-for-sale, total securities in a loss position, fair value | 10,975 | 10,623
Debt Securities, Available-for-sale, Unrealized Loss |  |
Available-for-sale, less than 12 continuous months, unrealized loss | 0 | 0
Available-for-sale, greater than 12 continuous months, unrealized loss | (962) | (1,300)
Available-for-sale, total securities in a loss position, unrealized loss | (962) | (1,300)
Obligations of state and political subdivisions |  |
Debt Securities, Held-to-maturity, Fair Value |  |
Less than 12 continuous months | 4,057 | 0
Greater than or equal to 12 continuous months | 45,312 | 44,437
Total securities in a loss position | 49,369 | 44,437
Debt Securities, Held-to-maturity, Unrealized Loss |  |
Less than 12 continuous months | (1) | 0
Greater than or equal to 12 continuous months | (6,697) | (7,884)
Held-to-maturity, gross unrealized losses | (6,698) | (7,884)
Debt Securities, Available-For-Sale, Fair Value |  |
Available-for-sale, less than 12 continuous months, fair value | 0 | 666
Available-for-sale, greater than 12 continuous months, fair value | 86,355 | 90,655
Available-for-sale, total securities in a loss position, fair value | 86,355 | 91,321
Debt Securities, Available-for-sale, Unrealized Loss |  |
Available-for-sale, less than 12 continuous months, unrealized loss | 0 | (1)
Available-for-sale, greater than 12 continuous months, unrealized loss | (9,427) | (10,320)
Available-for-sale, total securities in a loss position, unrealized loss | (9,427) | (10,321)
Corporate bonds |  |
Debt Securities, Held-to-maturity, Fair Value |  |
Less than 12 continuous months |  | 0
Greater than or equal to 12 continuous months | 29,373 | 28,804
Total securities in a loss position | 29,373 | 28,804
Debt Securities, Held-to-maturity, Unrealized Loss |  |
Less than 12 continuous months |  | 0
Greater than or equal to 12 continuous months | (627) | (1,196)
Held-to-maturity, gross unrealized losses | (627) | (1,196)
Debt Securities, Available-For-Sale, Fair Value |  |
Available-for-sale, less than 12 continuous months, fair value | 0 | 0
Available-for-sale, greater than 12 continuous months, fair value | 5,578 | 10,718
Available-for-sale, total securities in a loss position, fair value | 5,578 | 10,718
Debt Securities, Available-for-sale, Unrealized Loss |  |
Available-for-sale, less than 12 continuous months, unrealized loss | 0 | 0
Available-for-sale, greater than 12 continuous months, unrealized loss | (422) | (1,274)
Available-for-sale, total securities in a loss position, unrealized loss | $ (422) | $ (1,274)

X
- Definition Available-for-sale Securities and Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value, Total + References No definition available. + Details
Name: | bmrc_AvailableForSaleSecuritiesAndHeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValueTotal
Namespace Prefix: | bmrc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Available-for-sale Securities and Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value + References No definition available. + Details
Name: | bmrc_AvailableForSaleSecuritiesAndHeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Namespace Prefix: | bmrc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Available-for-sale Securities and Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value + References No definition available. + Details
Name: | bmrc_AvailableForSaleSecuritiesAndHeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Namespace Prefix: | bmrc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Available-for-sale Securities and Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss + References No definition available. + Details
Name: | bmrc_AvailableforsaleSecuritiesandHeldtomaturitySecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAccumulatedLoss
Namespace Prefix: | bmrc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Available-for-sale Securities and Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss + References No definition available. + Details
Name: | bmrc_AvailableforsaleSecuritiesandHeldtomaturitySecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
Namespace Prefix: | bmrc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Available-for-sale Securities and Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss + References No definition available. + Details
Name: | bmrc_AvailableforsaleSecuritiesandHeldtomaturitySecuritiesContinuousUnrealizedLossPositionLessthan12MonthsAccumulatedLoss
Namespace Prefix: | bmrc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Debt Securities, Available-For-Sale, Continuous Unrealized Loss Position, Fair Value [Abstract] + References No definition available. + Details
Name: | bmrc_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionFairValueAbstract
Namespace Prefix: | bmrc_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Marketable Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract] + References No definition available. + Details
Name: | bmrc_MarketableSecuritiesContinuousUnrealizedLossPositionAccumulatedLossAbstract
Namespace Prefix: | bmrc_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Marketable Securities, Continuous Unrealized Loss Position, Fair Value [Abstract] + References No definition available. + Details
Name: | bmrc_MarketableSecuritiesContinuousUnrealizedLossPositionFairValueAbstract
Namespace Prefix: | bmrc_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLossAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 55 -Paragraph 8 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479081/326-30-55-8 Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 7 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-7 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479106/326-30-50-5 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset. + References Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 7 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-7 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479106/326-30-50-5 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 55 -Paragraph 8 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479081/326-30-55-8 Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 7 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-7 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479106/326-30-50-5 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset. + References Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 7 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-7 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479106/326-30-50-5 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position without allowance for credit loss. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 55 -Paragraph 8 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479081/326-30-55-8 Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (a)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-6 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 4 -Subparagraph (a)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479106/326-30-50-4 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset. + References Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (a)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-6 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 4 -Subparagraph (a)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479106/326-30-50-4 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity), in continuous loss position for 12 months or longer. + References Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 7 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-7 Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (a)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-6 + Details
Name: | us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity), in unrealized loss position. + References Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (a)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-6 + Details
Name: | us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionAccumulatedLossAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Fair value of investment in debt security measured at amortized cost (held-to-maturity), in unrealized loss position. + References Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (a)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-6 + Details
Name: | us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValueAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity), in continuous loss position for less than 12 months. + References Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 7 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-7 Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (a)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-6 + Details
Name: | us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Fair value of investment in debt security measured at amortized cost (held-to-maturity), in continuous unrealized loss position for less than 12 months. + References Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 7 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-7 + Details
Name: | us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Fair value of investment in debt security measured at amortized cost (held-to-maturity), in continuous unrealized loss position for 12 months or longer. + References Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 7 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-7 + Details
Name: | us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_CollateralizedMortgageObligationsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=bmrc_SmallBusinessAdministrationSBABackedSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_USTreasurySecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_CorporateBondSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |