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Note 4 - Investments - Cash Flows (Details) - USD ($) $ in Millions | 6 Months Ended
--- | ---
Jun. 30, 2025 | Jun. 30, 2024
Investments, net | $ (166.4) | $ (42.2)
Seeded Investment Products [Member] |  |
Purchases and settlements | (348.1) | (103.5)
Sales, settlements and maturities | 40.4 | 39.4
Investments, net | (307.7) | (64.1)
Securities Investment [Member] |  |
Purchases and settlements | (267.8) | (114.8)
Sales, settlements and maturities | 101.4 | 72.6
Investments, net | $ (166.4) | $ (42.2)

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: |