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Note 8 - Debt - Schedule of Debt (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Senior Notes, 5.450 Percent, Due 2034 [Member]
Long-term debt	$ 395.4	$ 395.0

X

- Definition

Amount, after deduction of unamortized premium (discount) and debt issuance cost, of long-term debt. Excludes lease obligation.

+ References

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+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Details

Name:	us-gaap_DebtInstrumentAxis=jhg_SeniorNotes5450PercentDue2034Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: