Total net investment income and net realized and unrealized investment gains (losses) - Net Realized Investment 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 realized investment losses on Other long-term investments | $ (3.4) | $ (19.6) | $ (8.8) | $ (23.3) |
| Other long-term investments | ||||
| Net Investment Income [Line Items] | ||||
| Net realized investment losses on Other long-term investments | (0.2) | (9.3) | (2.3) | (10.1) |
| Hedge funds and private equity funds | ||||
| Net Investment Income [Line Items] | ||||
| Net realized investment losses on Other long-term investments | (0.2) | (1.3) | (1.5) | (0.3) |
| Strategic investments | ||||
| Net Investment Income [Line Items] | ||||
| Net realized investment losses on Other long-term investments | $ 0.0 | $ (8.0) | $ (0.8) | $ (9.8) |