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Note 4 - Investments - Cash Flows (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024
Investments, net	$ (140.2)	$ (94.2)
Seeded Investment Products [Member]
Purchases and settlements	(397.8)	(230.2)
Sales, settlements and maturities	140.5	51.2
Investments, net	(257.3)	(179.0)
Securities Investment [Member]
Purchases and settlements	(361.5)	(178.1)
Sales, settlements and maturities	221.3	83.9
Investments, net	$ (140.2)	$ (94.2)

X

- Definition

The net cash outflow from trading, available-for-sale securities and held-to-maturity securities and derivatives.

+ References

No definition available.

+ Details

Name:	jhg_ProceedsFromPaymentsForMarketableSecuritiesAndDerivatives
Namespace Prefix:	jhg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

The cash inflow associated with the aggregate amount received by the entity through sales, settlements or maturities of marketable securities and derivatives (trading, held-to-maturity, or available-for-sale) during the period.

+ References

No definition available.

+ Details

Name:	jhg_ProceedsFromSalesSettlementsOrMaturitiesOfMarketableSecuritiesAndDerivatives
Namespace Prefix:	jhg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

The cash outflow from purchases, settlements or maturities of trading, available-for-sale securities and held-to-maturity securities and derivatives.

+ References

No definition available.

+ Details

Name:	jhg_PurchasesSettlementsAndMaturitiesOfMarketableSecuritiesAndDerivatives
Namespace Prefix:	jhg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Details

Name:	us-gaap_InvestmentTypeAxis=jhg_SeededInvestmentProductsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_SecuritiesInvestmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: