Allowance for Loan Credit Losses - Additional information (Details) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2022 loan	Dec. 31, 2022 USD ($) loan
Allowance for Loan Credit Losses
Recorded investment in TDR's \| $		$ 418
Number of loans modified in TDRs \| loan	0
Number of financing receivable contracts \| loan		0
Additional commitments funds \| $		$ 0

Additional commitment to disburse funds on financing receivable contracts modified as troubled debt restructuring.

No definition available.

Name:	jmsb_FinancingReceivableTroubledDebtRestructuringAdditionalCommitmentToDisburseFunds
Namespace Prefix:	jmsb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Number of financing receivable contracts modified as troubled debt restructuring, that require special reserve.

+ References

No definition available.

+ Details

Name:	jmsb_FinancingReceivableTroubledDebtRestructuringNumberOfContractsThatRequireSpecialReserve
Namespace Prefix:	jmsb_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Accumulated amortized cost of financing receivable modified for debtor experiencing financial difficulty.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-42

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 33
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-33

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 310
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481746/310-40-50-4

+ Details

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

- Definition

Number of financing receivable contracts modified as troubled debt restructuring within previous 12 months, with subsequent payment default.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 34
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-34

+ Details

Name:	us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

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