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Loss and loss adjustment expense reserves - Activity in the Loss and Loss Adjustment Expense Reserves (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]    
Gross reserves for loss and loss adjustment expenses, beginning of period $ 5,653.9 $ 5,608.1
Less: loss and loss adjustment expenses recoverable, beginning of period (2,315.3) (2,295.1)
Less: deferred gains on retroactive reinsurance contracts 8.5 27.5
Net reserves for loss and loss adjustment expenses, beginning of period 3,347.1 3,340.5
Increase (decrease) in net loss and loss adjustment expenses incurred in respect of losses occurring in:    
Current year 436.0 356.4
Prior years (34.2) (38.9)
Loss and loss adjustment expenses incurred, net 401.8 317.5
Net loss and loss adjustment expenses paid in respect of losses occurring in:    
Current year (123.6) (44.0)
Prior years (201.9) (272.8)
Total net paid losses (325.5) (316.8)
Foreign currency translation 10.1 15.8
Net reserves for loss and loss adjustment expenses, end of period 3,433.5 3,357.0
Plus: loss and loss adjustment expenses recoverable, end of period 2,335.7 2,233.8
Plus: deferred gains on retroactive reinsurance (6.6) (25.5)
Gross reserves for loss and loss adjustment expenses, end of period $ 5,762.6 $ 5,565.3