Deposits and Borrowings - Time deposits and FHLB advances by contractual maturity (Details) $ in Thousands	Sep. 30, 2023 USD ($)
Contractual maturity
2023	$ 146,873
2024	477,572
2025	144,346
2026	40,324
2027	1,117
Thereafter	284
Total	$ 810,516

Amount of time deposit liabilities and Fhlp advances.

No definition available.

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Amount of time deposit and Fhlp advances maturing after fourth fiscal year following current fiscal year.

No definition available.

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Data Type:	xbrli:monetaryItemType
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Period Type:	instant

Amount of time deposit and Fhlp advances maturing in next twelve months.

No definition available.

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Namespace Prefix:	jmsb_
Data Type:	xbrli:monetaryItemType
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Amount of time deposit and Fhlp advances maturing in remainder of current fiscal year.

No definition available.

Name:	jmsb_TimeDepositsAndFhlbAdvancesRemainderOfFiscalYear
Namespace Prefix:	jmsb_
Data Type:	xbrli:monetaryItemType
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Period Type:	instant

Amount of time deposit and Fhlp advances maturing in fourth fiscal year following current fiscal year.

No definition available.

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Amount of time deposit and Fhlp advances maturing in third fiscal year following current fiscal year

No definition available.

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Data Type:	xbrli:monetaryItemType
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Amount of time deposit and Fhlp advances maturing in second fiscal year following current fiscal year.

No definition available.

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

No definition available.

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Period Type:	duration