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Note 16 - Retirement Benefit Plans (Tables) | 12 Months Ended
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Dec. 31, 2024
Notes Tables |
Defined Benefit Plan, Plan with Projected Benefit Obligation in Excess of Plan Assets [Table Text Block] |
​ |  | 2024 |  |  | 2023 |
Change in plan assets: |  |  |  |  |  |  |  |
Fair value of plan assets as of January 1 |  | $ | 696.4 |  |  | $ | 660.5 |
Return on plan assets |  |  | (32.9 | ) |  |  | 15.2 |
Employer contributions |  |  | 2.4 |  |  |  | 2.2 |
Benefits paid |  |  | (19.5 | ) |  |  | (17.9 | )
Settlements |  |  | (9.6 | ) |  |  | (3.2 | )
Foreign currency translation |  |  | (11.7 | ) |  |  | 39.6 |
Fair value of plan assets as of December 31 |  |  | 625.1 |  |  |  | 696.4 |
Change in benefit obligation: |  |  |  |  |  |  |  |
Benefit obligation as of January 1 |  |  | (611.1 | ) |  |  | (565.6 | )
Interest cost |  |  | (27.2 | ) |  |  | (27.6 | )
Settlements |  |  | 9.6 |  |  |  | 3.2 |
Benefits paid |  | 19.5 |  |  |  | 17.9 |
Actuarial gain/(loss) |  |  | 42.1 |  |  |  | (5.1 | )
Foreign currency translation |  |  | 10.3 |  |  |  | (33.9 | )
Benefit obligation as of December 31 |  |  | (556.8 | ) |  |  | (611.1 | )
Funded status as of year-end |  |  | 68.3 |  |  |  | 85.3 |
Net retirement benefit asset recognized in the Consolidated Balance Sheets |  | $ | 68.3 |  |  | $ | 85.3 |
Schedule of Amounts Recognized in Balance Sheet [Table Text Block] |
​ |  | December 31, |
​ |  | 2024 |  |  | 2023 |
Retirement benefit assets recognized in the Consolidated Balance Sheets: |  |  |  |  |  |  |  |
Janus Henderson Group UK Pension Scheme |  | $ | 70.3 |  |  | $ | 87.6 |
Retirement benefit obligations recognized in the Consolidated Balance Sheets: |  | ​ |  |  | ​ |
Janus Henderson Group unapproved pension scheme |  |  | (2.0 | ) |  |  | (2.3 | )
Net retirement benefit asset recognized in the Consolidated Balance Sheets |  | $ | 68.3 |  |  | $ | 85.3 |
Schedule of Assumptions Used in Determining Defined Benefit Obligation [Table Text Block] |
​ |  | December 31, |
​ |  | 2024 |  |  | 2023 |
Discount rate |  |  | 5.5 | % |  |  | 4.5 | %
Inflation — Retail Price Index (“RPI”) |  |  | 3.2 | % |  |  | 3.1 | %
Inflation — Consumer Price Index (“CPI”) |  |  | 2.5 | % |  |  | 2.5 | %
Pension increases (RPI capped at 5% per annum (“p.a.”)) |  |  | 3.1 | % |  |  | 3.0 | %
Pension increases (RPI capped at 2.5% p.a.) |  |  | 2.1 | % |  |  | 2.1 | %
Life expectancy of male aged 60 at accounting date |  |  | 29.0 |  |  |  | 29.1 |
Life expectancy of male aged 60 in 15 years’ time |  |  | 30.0 |  |  |  | 30.1 |
Schedule of Allocation of Plan Assets [Table Text Block] |
​ |  | December 31, |
​ |  | 2024 |  |  | 2023 |
Cash and cash equivalents |  | $ | 2.9 |  |  | $ | 19.9 |
Money market instruments |  |  | 7.1 |  |  |  | 7.8 |
Bulk annuity policy |  |  | 217.7 |  |  |  | 240.7 |
Fixed income investments |  |  | 269.0 |  |  |  | 300.6 |
Equity investments |  |  | 128.4 |  |  |  | 127.4 |
Total assets at fair value |  | $ | 625.1 |  |  | $ | 696.4 |
Schedule of Defined Benefit Plans Disclosures [Table Text Block] |
​ |  | Fair value measurements using: |  |  | ​ |
​ |  | Quoted prices in |  |  | Significant |  |  | ​ |  |  | ​ |
​ |  | active markets for |  |  | other |  |  | Significant |  |  | ​ |
​ |  | identical assets |  |  | observable |  |  | unobservable |  |  | ​ |
​ |  | and liabilities |  |  | inputs |  |  | inputs |  |  | ​ |
​ |  | (Level 1) |  |  | (Level 2) |  |  | (Level 3) |  |  | Total |
Cash and cash equivalents |  | $ | 2.9 |  |  | $ | — |  |  | $ | — |  |  | $ | 2.9 |
Money market instruments |  |  | 7.1 |  |  |  | — |  |  |  | — |  |  |  | 7.1 |
Bulk annuity contract |  |  | — |  |  |  | — |  |  |  | 217.7 |  |  |  | 217.7 |
Fixed income investments |  |  | 269.0 |  |  |  | — |  |  |  | — |  |  |  | 269.0 |
Equity investments |  |  | 128.4 |  |  |  | — |  |  |  | — |  |  |  | 128.4 |
Total |  | $ | 407.4 |  |  | $ | — |  |  | $ | 217.7 |  |  | $ | 625.1 |
​ |  | Fair value measurements using: |  |  | ​ |
​ |  | Quoted prices in |  |  | Significant |  |  | ​ |  |  | ​ |
​ |  | active markets for |  |  | other |  |  | Significant |  |  | ​ |
​ |  | identical assets |  |  | observable |  |  | unobservable |  |  | ​ |
​ |  | and liabilities |  |  | inputs |  |  | inputs |  |  | ​ |
​ |  | (Level 1) |  |  | (Level 2) |  |  | (Level 3) |  |  | Total |
Cash and cash equivalents |  | $ | 19.9 |  |  | $ | — |  |  | $ | — |  |  | $ | 19.9 |
Money market instruments |  |  | 7.8 |  |  |  | — |  |  |  | — |  |  |  | 7.8 |
Bulk annuity contract |  |  | — |  |  |  | — |  |  |  | 240.7 |  |  |  | 240.7 |
Fixed income investments |  |  | 300.6 |  |  |  | — |  |  |  | — |  |  |  | 300.6 |
Equity investments |  |  | 127.4 |  |  |  | — |  |  |  | — |  |  |  | 127.4 |
Total |  | $ | 455.7 |  |  | $ | — |  |  | $ | 240.7 |  |  | $ | 696.4 |
Schedule of Actuarial Gains and Losses [Table Text Block] |
​ |  | 2024 |  |  | 2023 |
Opening accumulated unamortized actuarial loss as of January 1 |  | $ | (84.7 | ) |  | $ | (70.4 | )
Actuarial loss |  |  | (17.5 | ) |  |  | (18.8 | )
Tax at source on current year actuarial gain |  |  | 3.2 |  |  |  | 3.5 |
Prior service cost |  |  | 0.4 |  |  |  | 0.5 |
Amortization of actuarial loss |  |  | 1.3 |  |  |  | 0.5 |
Closing accumulated unamortized actuarial loss as of December 31 |  | $ | (97.3 | ) |  | $ | (84.7 | )
Schedule of Assumptions Used in Determining Net Periodic Benefit Cost [Table Text Block] |
​ |  | Year ended December 31, |
​ |  | 2024 |  |  | 2023 |  |  | 2022 |
Interest cost |  | $ | (27.2 | ) |  | $ | (27.6 | ) |  | $ | (16.9 | )
Amortization of actuarial loss |  |  | (1.3 | ) |  |  | (0.5 | ) |  |  | — |
Amortization of prior service cost |  |  | (0.4 | ) |  |  | (0.4 | ) |  |  | (0.4 | )
Expected return on plan assets |  |  | 26.7 |  |  |  | 28.7 |  |  |  | 13.8 |
Net periodic benefit credit (cost) |  |  | (2.2 | ) |  |  | 0.2 |  |  |  | (3.5 | )
Contributions to money purchase section |  |  | (12.5 | ) |  |  | (11.6 | ) |  |  | (11.5 | )
Total cost |  | $ | (14.7 | ) |  | $ | (11.4 | ) |  | $ | (15.0 | )
​ |  | Year ended December 31, |
​ |  | 2024 |  |  | 2023 |  |  | 2022 |
Discount rate |  |  | 4.5 | % |  |  | 4.8 | % |  |  | 1.9 | %
Inflation — RPI |  |  | 3.1 | % |  |  | 3.3 | % |  |  | 3.4 | %
Inflation — CPI |  |  | 2.5 | % |  |  | 2.7 | % |  |  | 2.8 | %
Pension increases (RPI capped at 5% p.a.) |  |  | 3.0 | % |  |  | 3.2 | % |  |  | 3.3 | %
Pension increases (RPI capped at 2.5% p.a.) |  |  | 2.1 | % |  |  | 2.1 | % |  |  | 2.2 | %
Expected return on plan assets |  |  | 4.1 | % |  |  | 4.4 | % |  |  | 1.6 | %
Amortization period for net actuarial gains at beginning of the year |  |  | 27.0 |  |  |  | 28.0 |  |  |  | 29.0 |
Schedule of Expected Benefit Payments [Table Text Block] |
2025 |  |  | $ | 21.7 |
2026 |  |  | $ | 22.9 |
2027 |  |  | $ | 24.0 |
2028 |  |  | $ | 24.7 |
2029 |  |  | $ | 26.2 |
2030-2034 |  |  | $ | 138.5 |

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