Total net investment income and net realized and unrealized investment gains (losses) - Gains Attributable to Unrealized Investment Gains on Level 3 Investments (Details) - USD ($) $ in Millions |
3 Months Ended | 6 Months Ended | ||
|---|---|---|---|---|
Jun. 30, 2025 |
Jun. 30, 2024 |
Jun. 30, 2025 |
Jun. 30, 2024 |
|
| Net Investment Income [Line Items] | ||||
| Other long-term investments | $ 1.1 | $ (39.6) | $ 3.7 | $ (38.3) |
| Level 3 | ||||
| Net Investment Income [Line Items] | ||||
| Other long-term investments | $ (1.3) | $ (44.7) | $ 0.1 | $ (43.6) |