Total net investment income and net realized and unrealized investment gains (losses) - Net Realized Investment Losses (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] | ||||
| Debt securities, available for sale | $ (2.4) | $ (7.1) | $ (4.4) | $ (7.4) |
| Debt securities, trading | (0.3) | (3.7) | (1.8) | (7.1) |
| Short-term investments | 0.0 | 0.1 | 0.2 | 0.7 |
| Other long-term investments | (0.2) | (9.3) | (2.3) | (10.1) |
| Net investment income (loss) on cash and cash equivalents | (0.5) | 0.4 | (0.5) | 0.6 |
| Net realized losses on investments | $ (3.4) | $ (19.6) | $ (8.8) | $ (23.3) |