v3.26.3
Loss and loss adjustment expense reserves - Activity in the Loss and Loss Adjustment Expense Reserves (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]        
Gross reserves for loss and loss adjustment expenses, beginning of period     $ 5,782.5 $ 5,653.9
Less: loss and loss adjustment expenses recoverable, beginning of period     (2,102.3) (2,315.3)
Less: deferred gains (charges) on retroactive reinsurance contracts     0.0 8.5
Net reserves for loss and loss adjustment expenses, beginning of period     3,680.2 3,347.1
Increase (decrease) in net loss and loss adjustment expenses incurred in respect of losses occurring in:        
Current year     1,189.7 1,199.1
Prior years $ (19.8) $ (8.9) (52.7) (51.8)
Total incurred loss and loss adjustment expenses 415.9 372.9 1,137.0 1,147.3
Net loss and loss adjustment expenses paid in respect of losses occurring in:        
Current year     (332.6) (445.2)
Prior years     (622.8) (451.8)
Total net paid losses     (955.4) (897.0)
Foreign currency translation     (11.5) 51.4
Net reserves for loss and loss adjustment expenses, end of period 3,850.3 3,648.8 3,850.3 3,648.8
Plus: loss and loss adjustment expenses recoverable, end of period 1,966.4 2,162.9 1,966.4 2,162.9
Gross reserves for loss and loss adjustment expenses, end of period $ 5,816.7 $ 5,811.7 $ 5,816.7 $ 5,811.7