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Note 7 - Goodwill and Intangible Assets (Details Textual) $ in Millions | Jun. 30, 2025 USD ($)
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Finite-Lived Intangible Assets, Net | $ 92.7
Guardian Life Insurance Company [Member] | Strategic Partnership [Member] |
Finite-Lived Intangible Assets, Net | $ 41.1

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- 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

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

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