Total net investment income and net realized and unrealized investment gains (losses) - Net Unrealized Investment Gains (Losses) on Other Long-Term 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] | ||||
| Net unrealized investment gains (losses) on Other long-term investments | $ 1.1 | $ (39.6) | $ 3.7 | $ (38.3) |
| Hedge funds and private equity funds | ||||
| Net Investment Income [Line Items] | ||||
| Net unrealized investment gains (losses) on Other long-term investments | 1.5 | 5.3 | 3.5 | 4.7 |
| Strategic investments | ||||
| Net Investment Income [Line Items] | ||||
| Net unrealized investment gains (losses) on Other long-term investments | (0.4) | (35.6) | 0.1 | (33.7) |
| Other investments | ||||
| Net Investment Income [Line Items] | ||||
| Net unrealized investment gains (losses) on Other long-term investments | $ 0.0 | $ (9.3) | $ 0.1 | $ (9.3) |