Allowance for Loan Credit Losses - Credit quality indicators (Details) - USD ($) $ in Thousands	Jun. 30, 2025	Dec. 31, 2024
Financing receivable, credit quality indicator
Total	$ 1,916,915	$ 1,872,173
Commercial Real Estate \| Real Estate Loans
Financing receivable, credit quality indicator
Current year	64,607	133,591
Prior year	145,068	66,453
Two years prior	66,343	299,883
Three years prior	285,567	186,402
Four years prior	163,887	116,364
Prior	462,777	376,923
Revolving	3,818	1,474
Total	1,192,067	1,181,090
Construction and land development \| Real Estate Loans
Financing receivable, credit quality indicator
Current year	27,579	64,826
Prior year	71,594	40,190
Two years prior	27,318	17,635
Three years prior	15,703	4,395
Four years prior	4,598	2,254
Prior	14,696	13,075
Revolving	24,921	22,613
Total	186,409	164,988
Residential Real Estate \| Real Estate Loans
Financing receivable, credit quality indicator
Current year	38,816	31,815
Prior year	29,644	71,489
Two years prior	65,053	110,724
Three years prior	105,474	114,991
Four years prior	111,983	81,482
Prior	115,605	39,868
Revolving	22,947	22,563
Total	489,522	472,932
Commercial \| Commercial - Non-Real Estate
Financing receivable, credit quality indicator
Current year	5,284	13,622
Prior year	5,775	4,628
Two years prior	3,936	5,770
Three years prior	5,223	1,351
Four years prior	857	1,323
Prior	7,286	7,032
Revolving	14,921	14,010
Total	43,282	47,736
Consumer \| Consumer - Non-Real Estate
Financing receivable, credit quality indicator
Current year	403	812
Prior year	537	72
Two years prior	42
Prior		3
Revolving	16	19
Total	998	906
Pass \| Commercial Real Estate \| Real Estate Loans
Financing receivable, credit quality indicator
Current year	64,607	133,591
Prior year	145,068	66,453
Two years prior	66,343	287,181
Three years prior	263,937	176,424
Four years prior	163,887	116,364
Prior	454,027	362,135
Revolving	3,818	1,474
Total	1,161,687	1,143,622
Pass \| Construction and land development \| Real Estate Loans
Financing receivable, credit quality indicator
Current year	27,579	64,826
Prior year	71,594	40,190
Two years prior	27,318	17,635
Three years prior	15,703	4,395
Four years prior	4,598	2,254
Prior	13,605	11,974
Revolving	24,921	22,613
Total	185,318	163,887
Pass \| Residential Real Estate \| Real Estate Loans
Financing receivable, credit quality indicator
Current year	38,816	31,815
Prior year	29,644	71,489
Two years prior	65,053	110,724
Three years prior	105,474	114,991
Four years prior	111,983	81,482
Prior	115,605	39,868
Revolving	22,947	22,563
Total	489,522	472,932
Pass \| Commercial \| Commercial - Non-Real Estate
Financing receivable, credit quality indicator
Current year	5,284	13,622
Prior year	5,775	4,628
Two years prior	3,936	5,770
Three years prior	5,223	1,351
Four years prior	857	1,323
Prior	7,286	7,032
Revolving	14,921	14,010
Total	43,282	47,736
Pass \| Consumer \| Consumer - Non-Real Estate
Financing receivable, credit quality indicator
Current year	403	812
Prior year	537	72
Two years prior	42
Prior		3
Revolving	16	19
Total	998	906
Special Mention \| Commercial Real Estate \| Real Estate Loans
Financing receivable, credit quality indicator
Two years prior		12,702
Three years prior	21,630
Prior	8,750	14,788
Total	30,380	27,490
Special Mention \| Construction and land development \| Real Estate Loans
Financing receivable, credit quality indicator
Prior	1,091	1,101
Total	$ 1,091	1,101
Substandard \| Commercial Real Estate \| Real Estate Loans
Financing receivable, credit quality indicator
Three years prior		9,978
Total		$ 9,978

Amortized cost excluding accrued interest, before allowance for credit loss, of financing receivable. Excludes net investment in lease.

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

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 3B
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-3B

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-5

+ Details

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

- Definition

Amortized cost excluding accrued interest, of financing receivable originated more than five years prior to current fiscal year. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 79
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479294/326-20-55-79

Reference 2: 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 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 3B
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-3B

+ Details

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

- Definition

Amortized cost excluding accrued interest, of financing receivable that can be withdrawn, repaid, and redrawn.

+ References

+ Details

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

- Definition

Amortized cost excluding accrued interest, of financing receivable originated four years prior to current fiscal year. Excludes net investment in lease.

+ References

+ Details

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

- Definition

Amortized cost excluding accrued interest, of financing receivable originated three years prior to current fiscal year. Excludes net investment in lease.

+ References

+ Details

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

- Definition

Amortized cost excluding accrued interest, of financing receivable originated in current fiscal year. Excludes net investment in lease.

+ References

+ Details

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

- Definition

Amortized cost excluding accrued interest, of financing receivable originated two years prior to current fiscal year. Excludes net investment in lease.

+ References

+ Details

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

- Definition

Amortized cost excluding accrued interest of financing receivable originated in fiscal year prior to current fiscal year. Excludes net investment in lease.

+ References

+ Details

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

- 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

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 29
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-29

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-5

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 79
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479294/326-20-55-79

+ Details

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

- Details

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

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=jmsb_RealEstatePortfolioSegmentMember
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_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConsumerLoanMember
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_InternalCreditAssessmentAxis=us-gaap_PassMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_SpecialMentionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_SubstandardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: