Variable and voting interest entities - Total Assets and Maximum Exposure to Loss for Unconsolidated Variable Interest Entities (Details) - Variable Interest Entity, Not Primary Beneficiary, Excluding Investments In Related Party Investment Funds - USD ($) $ in Millions |
Sep. 30, 2026 |
Dec. 31, 2025 |
|---|---|---|
| Variable Interest Entity [Line Items] | ||
| Carrying Amount | $ 285.7 | $ 264.2 |
| Maximum Exposure to Loss | 362.6 | 372.7 |
| Debt securities, available for sale | ||
| Variable Interest Entity [Line Items] | ||
| Carrying Amount | 78.1 | 54.1 |
| Maximum Exposure to Loss | 101.6 | 78.8 |
| Other long-term investments | ||
| Variable Interest Entity [Line Items] | ||
| Carrying Amount | 207.6 | 210.1 |
| Maximum Exposure to Loss | $ 261.0 | $ 293.9 |