| Cover | Apr. 29, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Apr. 29, 2021 |
| Entity File Number | 001-38103 |
| Entity Registrant Name | JANUS HENDERSON GROUP PLC |
| Entity Central Index Key | 0001274173 |
| Entity Tax Identification Number | 98-1376360 |
| Entity Incorporation, State or Country Code | Y9 |
| Entity Address, Address Line One | 201 Bishopsgate |
| Entity Address, City or Town | London |
| Entity Address, Country | GB |
| Entity Address, Postal Zip Code | EC2M3AE |
| Country Region | +44 |
| City Area Code | (0) 20 |
| Local Phone Number | 7818 1818 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, $1.50 Per Share Par Value |
| Trading Symbol | JHG |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
