Deposits and Borrowings - Time deposits and FHLB advances by contractual maturity (Details) $ in Thousands	Jun. 30, 2022 USD ($)
Contractual maturity
2022	$ 227,481
2023	384,886
2024	62,747
2025	2,967
2026	2,364
Thereafter	70
Total	$ 680,515

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

Amount of time deposit and Fhlp advances maturing in next 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 third 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 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