Segment Information - Revenues (Details) - USD ($) $ in Millions |
3 Months Ended | |
|---|---|---|
Mar. 31, 2025 |
Mar. 31, 2024 |
|
| Segment Reporting Information [Line Items] | ||
| Earned insurance premiums | $ 395.0 | $ 319.0 |
| Commission and fee revenues | 48.1 | 25.5 |
| Other revenues | 13.6 | 14.3 |
| Total | 456.7 | 358.8 |
| UNITED STATES | ||
| Segment Reporting Information [Line Items] | ||
| Earned insurance premiums | 14.9 | 1.3 |
| Commission and fee revenues | 44.2 | 21.9 |
| Other revenues | 13.6 | 14.3 |
| Total | 72.7 | 37.5 |
| UNITED KINGDOM | ||
| Segment Reporting Information [Line Items] | ||
| Earned insurance premiums | 215.5 | 177.4 |
| Commission and fee revenues | 0.0 | 0.0 |
| Other revenues | 0.0 | 0.0 |
| Total | 215.5 | 177.4 |
| BERMUDA | ||
| Segment Reporting Information [Line Items] | ||
| Earned insurance premiums | 164.6 | 140.3 |
| Commission and fee revenues | 0.0 | 0.0 |
| Other revenues | 0.0 | 0.0 |
| Total | 164.6 | 140.3 |
| Other country | ||
| Segment Reporting Information [Line Items] | ||
| Earned insurance premiums | 0.0 | 0.0 |
| Commission and fee revenues | 3.9 | 3.6 |
| Other revenues | 0.0 | 0.0 |
| Total | $ 3.9 | $ 3.6 |