Consolidation - VIEs (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Unconsolidated VIEs
Investment securities	$ 11.9	$ 9.9

Total debt and equity financial instruments including: (1) securities held-to-maturity, (2) trading securities, and (3) securities available-for-sale which are intended to be held for less than one year (4) derivative instruments which are expected to be converted into cash or otherwise disposed of within a year or the normal operating cycle, if longer.

No definition available.

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