Investments - Net Investment Losses (Details) - USD ($) $ in Millions |
3 Months Ended | 9 Months Ended | ||
|---|---|---|---|---|
Sep. 30, 2026 |
Sep. 30, 2025 |
Sep. 30, 2026 |
Sep. 30, 2025 |
|
| Gain (Loss) on Securities [Line Items] | ||||
| Net investment gains (losses) | $ (26.2) | $ 3.3 | $ (28.5) | $ (5.9) |
| Fixed maturity securities | ||||
| Gain (Loss) on Securities [Line Items] | ||||
| Net investment gains (losses) | (2.1) | (0.6) | (1.7) | (4.5) |
| Equity securities | ||||
| Gain (Loss) on Securities [Line Items] | ||||
| Net investment gains (losses) | (2.8) | 2.2 | (4.4) | 0.2 |
| Investment Portfolio | Limited partnership interests | ||||
| Gain (Loss) on Securities [Line Items] | ||||
| Net investment gains (losses) | (20.1) | 0.0 | (20.1) | 0.0 |
| Short-term investments and other | ||||
| Gain (Loss) on Securities [Line Items] | ||||
| Net investment gains (losses) | $ (1.2) | $ 1.7 | $ (2.3) | $ (1.6) |