Total net investment income and net realized and unrealized investment gains (losses) - Net Realized and Unrealized Gains (Losses) on Investments (Details) - USD ($) $ in Millions |
3 Months Ended | 6 Months Ended | ||
|---|---|---|---|---|
Jun. 30, 2025 |
Jun. 30, 2024 |
Jun. 30, 2025 |
Jun. 30, 2024 |
|
| Investments, Debt and Equity Securities [Abstract] | ||||
| Gross realized gains | $ 1.5 | $ 3.9 | $ 10.4 | $ 9.4 |
| Gross realized losses | (4.9) | (23.5) | (19.2) | (32.7) |
| Net realized losses on investments | (3.4) | (19.6) | (8.8) | (23.3) |
| Net unrealized gains (losses) on investments | 4.1 | (35.3) | 9.2 | (30.6) |
| Net realized and unrealized investment gains (losses) | 0.7 | (54.9) | 0.4 | (53.9) |
| Realized and unrealized gains (losses) from related party investments | $ (0.6) | $ 31.1 | $ 3.2 | $ 33.0 |