Segment Information - Revenues (Details) - USD ($) $ in Millions |
3 Months Ended | |
|---|---|---|
Mar. 31, 2026 |
Mar. 31, 2025 |
|
| Segment Reporting Information [Line Items] | ||
| Earned insurance premiums | $ 384.9 | $ 395.0 |
| Commission and fee revenues | 43.2 | 48.1 |
| Other revenues | 56.4 | 13.6 |
| Total | 484.5 | 456.7 |
| United States | ||
| Segment Reporting Information [Line Items] | ||
| Earned insurance premiums | 0.2 | 14.9 |
| Commission and fee revenues | 39.6 | 44.2 |
| Other revenues | 56.4 | 13.6 |
| Total | 96.2 | 72.7 |
| United Kingdom | ||
| Segment Reporting Information [Line Items] | ||
| Earned insurance premiums | 231.2 | 215.5 |
| Commission and fee revenues | 0.0 | 0.0 |
| Other revenues | 0.0 | 0.0 |
| Total | 231.2 | 215.5 |
| Bermuda | ||
| Segment Reporting Information [Line Items] | ||
| Earned insurance premiums | 153.5 | 164.6 |
| Commission and fee revenues | 0.0 | 0.0 |
| Other revenues | 0.0 | 0.0 |
| Total | 153.5 | 164.6 |
| Other | ||
| Segment Reporting Information [Line Items] | ||
| Earned insurance premiums | 0.0 | 0.0 |
| Commission and fee revenues | 3.6 | 3.9 |
| Other revenues | 0.0 | 0.0 |
| Total | $ 3.6 | $ 3.9 |