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Note 11 - Long-Term Incentive Compensation (Details Textual) - Deferred Incentive Plan [Member] | 6 Months Ended
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Jun. 30, 2025
Minimum [Member] |
Share-Based Compensation Arrangement by Share-Based Payment Award, Award Vesting Period (Year) | 3 years
Maximum [Member] |
Share-Based Compensation Arrangement by Share-Based Payment Award, Award Vesting Period (Year) | 5 years

X
- Definition Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_PlanNameAxis=jhg_DeferredIncentivePlanMember
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Data Type: | na
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- Details
Name: | srt_RangeAxis=srt_MinimumMember
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Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |