Commitments and Contingencies - Estimated Annual Undiscounted Future Minimum Lease Payments (Details) $ in Thousands	Dec. 31, 2021 USD ($)
Commitments and Contingencies Disclosure [Abstract]
2022	$ 0
2023	3,040
2024	3,412
2025	3,514
2026	3,620
Thereafter	24,115
Total	$ 37,701

Lessee operating lease liability payments not yet commenced.

No definition available.

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Namespace Prefix:	janx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Lessee operating lease liability payments not yet commenced due next five years.

No definition available.

Name:	janx_LesseeOperatingLeaseLiabilityPaymentsNotYetCommencedDueNextFiveYears
Namespace Prefix:	janx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Lessee operating lease liability payments not yet commenced due next four years.

No definition available.

Name:	janx_LesseeOperatingLeaseLiabilityPaymentsNotYetCommencedDueNextFourYears
Namespace Prefix:	janx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Lessee operating lease liability payments not yet commenced due next three years.

No definition available.

Name:	janx_LesseeOperatingLeaseLiabilityPaymentsNotYetCommencedDueNextThreeYears
Namespace Prefix:	janx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Lessee operating lease liability payments not yet commenced due next twelve months.

No definition available.

Name:	janx_LesseeOperatingLeaseLiabilityPaymentsNotYetCommencedDueNextTwelveMonths
Namespace Prefix:	janx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Lessee Operating Lease Liability Payments Not Yet Commenced Due Next Two Years

No definition available.

Name:	janx_LesseeOperatingLeaseLiabilityPaymentsNotYetCommencedDueNextTwoYears
Namespace Prefix:	janx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Lessee operating lease liability payments not yet commenced due thereafter.

No definition available.

Name:	janx_LesseeOperatingLeaseLiabilityPaymentsNotYetCommencedDueThereafter
Namespace Prefix:	janx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

No definition available.

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