Note 3 - Consolidation - Unconsolidated VREs (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Unconsolidated VREs [Member]
JHG’s net interest in consolidated VREs	$ 100.1	$ 73.5

The net carrying amount of the net interest in consolidated voting rights entities assets and liabilities excluding noncontrolling interest.

No definition available.

Name:	jhg_VotingRightsEntitiesConsolidatedAssetsAndLiabilitiesExcludingNoncontrollingInterestNet
Namespace Prefix:	jhg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant