v3.25.1
Reserves for Losses and Loss Adjustment Expenses - Schedule of Reserves for Losses and Loss Adjustment Expenses (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Dec. 31, 2024
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]      
Net reserves - beginning of the year $ 3,003.9 $ 2,747.1 $ 2,747.1
Losses and LAE incurred during current calendar year, net of reinsurance:      
Current accident year 147.1 195.2  
Prior accident years 1.0 24.8  
Losses and LAE incurred during calendar year, net of reinsurance 148.1 220.0  
Losses and LAE payments made during current calendar year, net of reinsurance:      
Current accident year 38.6 6.7  
Prior accident years 144.6 174.8  
Losses and LAE payments made during current calendar year, net of reinsurance: 183.2 181.5  
Add/(Deduct):      
Foreign exchange adjustments 0.0 (1.3)  
Net reserves - end of period 2,968.8 2,784.3 3,003.9
Reinsurance recoverables on unpaid losses and LAE, end of period 2,711.4 2,758.8  
Gross reserves - end of period $ 5,680.2 $ 5,543.1 $ 5,798.6