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Allowance for Loan Credit Losses - Allowance and loans by impairment methodology (Details) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2024 USD ($) loan	Dec. 31, 2023 loan
Allowance for Loan Credit Losses
Number of collateral dependent loans | loan	1	0
Individually evaluated for impairment, allowance | $	$ 10.0

X

- Definition

Number of collateral dependent loans.

+ References

No definition available.

+ Details

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

X

- References

No definition available.

+ Details

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

X

- Definition

Amount of allowance for credit loss on financing receivable evaluated on individual basis. Excludes financing receivable evaluated on collective basis.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 6.L.5.a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480418/310-10-S99-4

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 11B
-Subparagraph (g)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-11B

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 30
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479391/326-20-30-2

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant