Deposits and Borrowings - Time deposits and FHLB advances by contractual maturity (Details) $ in Thousands	Mar. 31, 2024 USD ($)
Contractual maturity
2024	$ 443,700
2025	241,722
2026	89,284
2027	25,292
2028	549
Thereafter	530
Total	$ 801,077

Amount of time deposit liabilities and Fhlb advances.

No definition available.

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

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 Fhlb 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 Fhlb 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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Amount of time deposit and Fhlb 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 Fhlb 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