Deposits and Borrowings - Time deposits and FHLB advances by contractual maturity (Details) $ in Thousands	Mar. 31, 2022 USD ($)
Contractual maturity
2022	$ 314,673
2023	238,509
2024	54,741
2025	2,111
2026	2,387
Thereafter	18,366
Total	$ 630,787

Amount of time deposit liabilities and Fhlp advances.

No definition available.

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

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

No definition available.

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

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

No definition available.

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

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

No definition available.

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

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

No definition available.

Name:	jmsb_TimeDepositsAndFhlbAdvancesYearThree
Namespace Prefix:	jmsb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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.

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