Collaborative arrangement milestone payments.

No definition available.

Name:	janx_CollaborativeArrangementMilestonePayments
Namespace Prefix:	janx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Developmental milestone.

+ References

No definition available.

+ Details

Name:	janx_DevelopmentalMilestone
Namespace Prefix:	janx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Milestone payments per collaboration target.

+ References

No definition available.

+ Details

Name:	janx_MilestonePaymentsPerCollaborationTarget
Namespace Prefix:	janx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Milestone payments upon successful completion of certain commercial milestones.

+ References

No definition available.

+ Details

Name:	janx_MilestonePaymentsUponSuccessfulCompletionOfCertainCommercialMilestones
Namespace Prefix:	janx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Non-refundable and non-creditable upfront fee.

+ References

No definition available.

+ Details

Name:	janx_NonRefundableAndNonCreditableUpfrontFee
Namespace Prefix:	janx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Number of Collaborative Arrangement Targets

+ References

No definition available.

+ Details

Name:	janx_NumberOfCollaborativeArrangementTargets
Namespace Prefix:	janx_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Payable upon selection of second collaboration target.

+ References

No definition available.

+ Details

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

- Definition

Amount of revenue recognized that was previously included in balance of obligation to transfer good or service to customer for which consideration from customer has been received or is due.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 8
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-8

+ Details

Name:	us-gaap_ContractWithCustomerLiabilityRevenueRecognized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 730
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483041/730-20-50-1

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 730
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483041/730-20-50-1

+ Details

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

- Details

Name:	us-gaap_TypeOfArrangementAxis=janx_MerckAgreementFirstCollaborationTargetMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
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Period Type:

Research Collaboration and Exclusive License Agreement with Merck - Additional Information (Detail) $ in Millions	1 Months Ended		12 Months Ended
	Aug. 31, 2025 USD ($)	Jun. 30, 2024 USD ($)	Dec. 31, 2025 USD ($) Target	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)
Merck Agreement First Collaboration Target
Research And Development Arrangement Contract To Perform For Others [Line Items]
Developmental milestone	$ 10.0	$ 7.5
Merck Agreement
Research And Development Arrangement Contract To Perform For Others [Line Items]
Number of collaboration targets \| Target			2
Non-refundable and non-creditable upfront fee			$ 8.0
Payable upon selection of second collaboration target			8.0
Milestone payments			285.0
Revenue recognized			10.0	$ 10.6	$ 8.1
Merck Agreement \| Maximum
Research And Development Arrangement Contract To Perform For Others [Line Items]
Milestone payments per collaboration target			142.5
Milestone payments upon successful completion of certain commercial milestones			$ 350.0