v3.26.3
Segment Information - Revenues (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Segment Reporting [Line Items]        
Earned insurance premiums $ 517.7 $ 557.8   $ 1,328.0
Earned insurance premiums     $ 1,287.2  
Commission and fee revenues     162.9  
Commission and fee revenues 59.3 82.3   193.7
Product and service revenues 121.1 68.8 285.1 138.7
Total 698.1 708.9 1,735.2 1,660.4
United States        
Segment Reporting [Line Items]        
Earned insurance premiums 0.3 5.2   21.7
Earned insurance premiums     0.7  
Commission and fee revenues     151.4  
Commission and fee revenues 55.2 78.0   181.3
Product and service revenues 121.1 68.8 285.1 138.7
Total 176.6 152.0 437.2 341.7
United Kingdom        
Segment Reporting [Line Items]        
Earned insurance premiums 329.0 335.2   779.7
Earned insurance premiums     797.8  
Commission and fee revenues     0.0  
Commission and fee revenues 0.0 0.0   0.0
Product and service revenues 0.0 0.0 0.0 0.0
Total 329.0 335.2 797.8 779.7
Bermuda        
Segment Reporting [Line Items]        
Earned insurance premiums 188.4 217.4   526.6
Earned insurance premiums     488.7  
Commission and fee revenues     0.0  
Commission and fee revenues 0.0 0.0   0.0
Product and service revenues 0.0 0.0 0.0 0.0
Total 188.4 217.4 488.7 526.6
Other        
Segment Reporting [Line Items]        
Earned insurance premiums 0.0 0.0   0.0
Earned insurance premiums     0.0  
Commission and fee revenues     11.5  
Commission and fee revenues 4.1 4.3   12.4
Product and service revenues 0.0 0.0 0.0 0.0
Total $ 4.1 $ 4.3 $ 11.5 $ 12.4