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Note 7 - Goodwill and Intangible Assets (Details Textual) $ in Millions	Sep. 30, 2025 USD ($)
Finite-Lived Intangible Assets, Net	$ 89.0
Guardian Life Insurance Company [Member] | Strategic Partnership [Member]
Finite-Lived Intangible Assets, Net	$ 41.1

X

- Definition

Amount after amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 350
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482665/350-30-50-2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 926
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483154/926-20-50-5

+ Details

Name:	us-gaap_FiniteLivedIntangibleAssetsNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=jhg_GuardianLifeInsuranceCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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