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Note 11 - Debt - Schedule of Debt (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Senior Notes, 4.876 Percent, Due 2025 [Member]
Long-term debt	$ 0.0	$ 304.6
Senior Notes, 5.450 Percent, Due 2034 [Member]
Long-term debt	$ 395.0	$ 0.0

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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-Topic 944
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-Name Accounting Standards Codification
-Section S99
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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_SeniorNotes4876PercentDue2025Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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