Deferred acquisition costs and deferred ceding commissions - Schedule of Ceding Commissions Deferred and Amortized (Details) - USD ($) $ in Millions |
3 Months Ended | 6 Months Ended | ||
|---|---|---|---|---|
Jun. 30, 2026 |
Jun. 30, 2025 |
Jun. 30, 2026 |
Jun. 30, 2025 |
|
| Movement in Deferred Revenue [Roll Forward] | ||||
| Beginning balance | $ 247.8 | $ 194.6 | $ 232.5 | $ 193.0 |
| Deferral of excess ceding commission income over deferred acquisition costs | 51.9 | 142.5 | 145.1 | 215.7 |
| Amortization of deferred excess ceding commission to income | (75.1) | (101.6) | (155.6) | (172.3) |
| Foreign currency translation | 0.5 | (0.5) | 3.1 | (1.4) |
| Deferred Income, Ending Balance | $ 225.1 | $ 235.0 | $ 225.1 | $ 235.0 |