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Loss and loss adjustment expense reserves - Loss and loss adjustment expense reserves by accident year and line of business (Details) - USD ($)
$ in Millions
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Claims Development [Line Items]                    
Net reserves for loss and allocated loss adjustment expenses, end of year $ 3,503.6                  
Reinsurance | Operating Segments | Aviation & Space                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 427.2                  
IBNR loss and ALAE reserves, net 45.8                  
Cumulative net losses and allocated loss adjustment expenses paid 294.8                  
Net reserves for loss and allocated loss adjustment expenses from 2012 to 2021 132.4                  
Net reserves for loss and allocated loss adjustment expenses prior to 2012 0.4                  
Net reserves for loss and allocated loss adjustment expenses, end of year 132.8                  
Reinsurance | Operating Segments | Aviation & Space | 2012                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 27.9 $ 28.0 $ 27.7 $ 28.7 $ 28.6 $ 27.5 $ 27.3 $ 29.3 $ 33.5 $ 34.7
IBNR loss and ALAE reserves, net (0.2)                  
Cumulative net losses and allocated loss adjustment expenses paid 28.1 27.9 27.5 27.8 27.3 26.4 23.7 21.8 17.6 7.0
Reinsurance | Operating Segments | Aviation & Space | 2013                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 31.8 31.1 31.4 31.3 31.2 30.3 31.6 34.7 38.7  
IBNR loss and ALAE reserves, net (0.3)                  
Cumulative net losses and allocated loss adjustment expenses paid 29.8 28.9 28.8 27.4 26.7 25.0 22.4 18.4 12.5  
Reinsurance | Operating Segments | Aviation & Space | 2014                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 29.7 30.4 28.8 30.3 30.5 31.5 34.9 32.3    
IBNR loss and ALAE reserves, net (2.8)                  
Cumulative net losses and allocated loss adjustment expenses paid 25.8 26.2 24.7 24.0 22.4 20.1 14.6 6.3    
Reinsurance | Operating Segments | Aviation & Space | 2015                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 34.8 33.9 33.7 34.4 35.4 31.4 35.1      
IBNR loss and ALAE reserves, net 1.2                  
Cumulative net losses and allocated loss adjustment expenses paid 33.8 33.2 32.4 30.9 24.5 18.5 9.0      
Reinsurance | Operating Segments | Aviation & Space | 2016                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 35.5 35.3 35.7 33.3 32.3 32.0        
IBNR loss and ALAE reserves, net (0.6)                  
Cumulative net losses and allocated loss adjustment expenses paid 33.6 32.4 29.0 26.6 19.5 7.7        
Reinsurance | Operating Segments | Aviation & Space | 2017                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 46.2 45.8 44.3 43.0 33.9          
IBNR loss and ALAE reserves, net (0.1)                  
Cumulative net losses and allocated loss adjustment expenses paid 39.5 35.9 32.6 23.7 9.0          
Reinsurance | Operating Segments | Aviation & Space | 2018                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 61.1 58.6 50.6 48.0            
IBNR loss and ALAE reserves, net 3.3                  
Cumulative net losses and allocated loss adjustment expenses paid 42.6 36.9 27.5 14.3            
Reinsurance | Operating Segments | Aviation & Space | 2019                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 76.4 72.2 60.7              
IBNR loss and ALAE reserves, net 6.9                  
Cumulative net losses and allocated loss adjustment expenses paid 32.8 22.7 8.4              
Reinsurance | Operating Segments | Aviation & Space | 2020                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 41.8 39.9                
IBNR loss and ALAE reserves, net 9.5                  
Cumulative net losses and allocated loss adjustment expenses paid 22.3 10.9                
Reinsurance | Operating Segments | Aviation & Space | 2021                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 42.0                  
IBNR loss and ALAE reserves, net 28.9                  
Cumulative net losses and allocated loss adjustment expenses paid 6.5                  
Reinsurance | Operating Segments | Casualty                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 2,592.7                  
IBNR loss and ALAE reserves, net 932.4                  
Cumulative net losses and allocated loss adjustment expenses paid 1,399.7                  
Net reserves for loss and allocated loss adjustment expenses from 2012 to 2021 1,193.0                  
Net reserves for loss and allocated loss adjustment expenses prior to 2012 21.5                  
Net reserves for loss and allocated loss adjustment expenses, end of year 1,214.5                  
Reinsurance | Operating Segments | Casualty | 2012                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 17.7 17.9 17.8 17.8 17.6 17.7 17.8 17.3 17.1 17.6
IBNR loss and ALAE reserves, net 0.0                  
Cumulative net losses and allocated loss adjustment expenses paid 17.4 17.4 17.1 16.7 16.3 16.6 16.0 14.5 10.7 5.7
Reinsurance | Operating Segments | Casualty | 2013                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 57.0 57.6 57.4 57.8 57.9 57.9 57.9 53.4 72.1  
IBNR loss and ALAE reserves, net 0.0                  
Cumulative net losses and allocated loss adjustment expenses paid 56.0 55.4 55.0 53.6 51.4 46.8 40.4 27.8 11.3  
Reinsurance | Operating Segments | Casualty | 2014                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 172.4 173.0 172.4 173.3 172.1 182.8 178.6 178.7    
IBNR loss and ALAE reserves, net 1.9                  
Cumulative net losses and allocated loss adjustment expenses paid 167.9 166.9 165.0 161.8 154.1 148.6 128.5 51.1    
Reinsurance | Operating Segments | Casualty | 2015                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 243.3 243.3 240.8 240.5 239.9 258.8 225.2      
IBNR loss and ALAE reserves, net 8.9                  
Cumulative net losses and allocated loss adjustment expenses paid 226.7 221.9 213.9 204.2 187.1 161.4 76.4      
Reinsurance | Operating Segments | Casualty | 2016                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 268.8 268.2 265.3 268.5 270.0 272.8        
IBNR loss and ALAE reserves, net 18.2                  
Cumulative net losses and allocated loss adjustment expenses paid 236.1 227.9 216.2 199.7 173.9 73.6        
Reinsurance | Operating Segments | Casualty | 2017                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 265.7 262.6 258.1 256.3 248.7          
IBNR loss and ALAE reserves, net 39.2                  
Cumulative net losses and allocated loss adjustment expenses paid 202.9 184.1 160.7 138.9 83.8          
Reinsurance | Operating Segments | Casualty | 2018                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 351.8 348.9 341.9 318.6            
IBNR loss and ALAE reserves, net 92.1                  
Cumulative net losses and allocated loss adjustment expenses paid 213.5 152.7 112.3 58.1            
Reinsurance | Operating Segments | Casualty | 2019                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 443.2 425.3 390.1              
IBNR loss and ALAE reserves, net 193.4                  
Cumulative net losses and allocated loss adjustment expenses paid 182.5 117.0 46.0              
Reinsurance | Operating Segments | Casualty | 2020                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 418.6 379.5                
IBNR loss and ALAE reserves, net 286.7                  
Cumulative net losses and allocated loss adjustment expenses paid 66.1 38.1                
Reinsurance | Operating Segments | Casualty | 2021                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 354.2                  
IBNR loss and ALAE reserves, net 292.0                  
Cumulative net losses and allocated loss adjustment expenses paid 30.6                  
Reinsurance | Operating Segments | Contingency                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 35.1                  
IBNR loss and ALAE reserves, net 10.3                  
Cumulative net losses and allocated loss adjustment expenses paid 18.5                  
Net reserves for loss and allocated loss adjustment expenses from 2012 to 2021 16.6                  
Net reserves for loss and allocated loss adjustment expenses prior to 2012 1.8                  
Net reserves for loss and allocated loss adjustment expenses, end of year 18.4                  
Reinsurance | Operating Segments | Contingency | 2012                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 0.1 0.2 0.2 0.2 0.2 0.2 0.2 0.1 0.2 0.3
IBNR loss and ALAE reserves, net 0.0                  
Cumulative net losses and allocated loss adjustment expenses paid 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
Reinsurance | Operating Segments | Contingency | 2013                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.1  
IBNR loss and ALAE reserves, net 0.1                  
Cumulative net losses and allocated loss adjustment expenses paid 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.1 0.0  
Reinsurance | Operating Segments | Contingency | 2014                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0    
IBNR loss and ALAE reserves, net 0.0                  
Cumulative net losses and allocated loss adjustment expenses paid 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0    
Reinsurance | Operating Segments | Contingency | 2015                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 0.0 0.0 0.0 0.0 0.0 0.0 0.0      
IBNR loss and ALAE reserves, net 0.0                  
Cumulative net losses and allocated loss adjustment expenses paid 0.0 0.0 0.0 0.0 0.0 0.0 0.0      
Reinsurance | Operating Segments | Contingency | 2016                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 0.0 0.0 0.0 0.0 0.0 0.0        
IBNR loss and ALAE reserves, net 0.0                  
Cumulative net losses and allocated loss adjustment expenses paid 0.0 0.0 0.0 0.0 0.0 0.0        
Reinsurance | Operating Segments | Contingency | 2017                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 1.0 1.0 1.3 0.7 0.7          
IBNR loss and ALAE reserves, net 0.1                  
Cumulative net losses and allocated loss adjustment expenses paid 0.8 0.8 0.6 0.3 0.0          
Reinsurance | Operating Segments | Contingency | 2018                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 1.8 1.7 2.0 1.6            
IBNR loss and ALAE reserves, net 0.2                  
Cumulative net losses and allocated loss adjustment expenses paid 1.6 1.5 0.9 0.0            
Reinsurance | Operating Segments | Contingency | 2019                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 2.0 2.0 2.0              
IBNR loss and ALAE reserves, net 0.2                  
Cumulative net losses and allocated loss adjustment expenses paid 1.3 1.2 (0.1)              
Reinsurance | Operating Segments | Contingency | 2020                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 20.1 21.3                
IBNR loss and ALAE reserves, net 3.8                  
Cumulative net losses and allocated loss adjustment expenses paid 10.3 7.2                
Reinsurance | Operating Segments | Contingency | 2021                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 9.7                  
IBNR loss and ALAE reserves, net 5.9                  
Cumulative net losses and allocated loss adjustment expenses paid 4.3                  
Reinsurance | Operating Segments | Credit & Bond                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 279.0                  
IBNR loss and ALAE reserves, net 43.1                  
Cumulative net losses and allocated loss adjustment expenses paid 206.7                  
Net reserves for loss and allocated loss adjustment expenses from 2012 to 2021 72.3                  
Net reserves for loss and allocated loss adjustment expenses prior to 2012 4.6                  
Net reserves for loss and allocated loss adjustment expenses, end of year 76.9                  
Reinsurance | Operating Segments | Credit & Bond | 2012                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 32.0 31.9 32.1 32.5 32.8 32.9 33.4 33.3 35.1 34.9
IBNR loss and ALAE reserves, net 0.1                  
Cumulative net losses and allocated loss adjustment expenses paid 31.8 32.0 32.1 32.1 32.1 32.2 31.9 31.1 26.9 14.8
Reinsurance | Operating Segments | Credit & Bond | 2013                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 29.1 28.6 28.5 28.3 28.7 27.8 28.2 29.3 30.6  
IBNR loss and ALAE reserves, net 0.6                  
Cumulative net losses and allocated loss adjustment expenses paid 25.5 25.3 25.2 25.1 24.9 24.2 23.0 19.9 11.6  
Reinsurance | Operating Segments | Credit & Bond | 2014                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 23.1 22.8 22.9 23.2 24.2 26.3 25.8 26.3    
IBNR loss and ALAE reserves, net (2.1)                  
Cumulative net losses and allocated loss adjustment expenses paid 21.7 21.6 21.6 21.3 20.6 18.6 14.1 7.9    
Reinsurance | Operating Segments | Credit & Bond | 2015                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 19.9 19.9 20.5 21.5 23.2 24.2 24.7      
IBNR loss and ALAE reserves, net 0.4                  
Cumulative net losses and allocated loss adjustment expenses paid 18.6 18.7 18.7 18.7 17.4 13.1 4.8      
Reinsurance | Operating Segments | Credit & Bond | 2016                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 17.0 16.7 17.1 18.0 18.5 20.2        
IBNR loss and ALAE reserves, net 0.8                  
Cumulative net losses and allocated loss adjustment expenses paid 15.4 15.1 14.7 13.6 10.7 5.5        
Reinsurance | Operating Segments | Credit & Bond | 2017                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 22.8 22.9 24.4 25.6 25.1          
IBNR loss and ALAE reserves, net 4.5                  
Cumulative net losses and allocated loss adjustment expenses paid 16.2 15.8 14.7 10.4 4.0          
Reinsurance | Operating Segments | Credit & Bond | 2018                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 31.2 31.1 32.1 31.7            
IBNR loss and ALAE reserves, net 3.5                  
Cumulative net losses and allocated loss adjustment expenses paid 24.6 23.3 17.8 8.3            
Reinsurance | Operating Segments | Credit & Bond | 2019                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 39.0 39.9 41.9              
IBNR loss and ALAE reserves, net 6.6                  
Cumulative net losses and allocated loss adjustment expenses paid 27.5 21.4 10.1              
Reinsurance | Operating Segments | Credit & Bond | 2020                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 41.6 45.6                
IBNR loss and ALAE reserves, net 14.5                  
Cumulative net losses and allocated loss adjustment expenses paid 20.9 18.8                
Reinsurance | Operating Segments | Credit & Bond | 2021                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 23.3                  
IBNR loss and ALAE reserves, net 14.2                  
Cumulative net losses and allocated loss adjustment expenses paid 4.5                  
Reinsurance | Operating Segments | Marine & Energy                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 222.0                  
IBNR loss and ALAE reserves, net 30.6                  
Cumulative net losses and allocated loss adjustment expenses paid 159.3                  
Net reserves for loss and allocated loss adjustment expenses from 2012 to 2021 62.7                  
Net reserves for loss and allocated loss adjustment expenses prior to 2012 2.1                  
Net reserves for loss and allocated loss adjustment expenses, end of year 64.8                  
Reinsurance | Operating Segments | Marine & Energy | 2012                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 18.9 18.9 18.9 19.1 19.2 19.2 18.9 18.2 17.0 15.3
IBNR loss and ALAE reserves, net 0.7                  
Cumulative net losses and allocated loss adjustment expenses paid 18.0 18.0 18.0 17.2 16.6 15.3 14.2 12.5 8.5 3.7
Reinsurance | Operating Segments | Marine & Energy | 2013                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 16.1 16.5 16.4 16.3 16.3 16.7 17.5 18.7 21.0  
IBNR loss and ALAE reserves, net 0.5                  
Cumulative net losses and allocated loss adjustment expenses paid 13.7 13.7 13.7 13.4 13.2 13.0 11.6 8.6 2.7  
Reinsurance | Operating Segments | Marine & Energy | 2014                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 18.2 18.2 18.2 17.5 18.1 19.4 21.1 22.4    
IBNR loss and ALAE reserves, net (0.2)                  
Cumulative net losses and allocated loss adjustment expenses paid 16.7 16.7 15.7 15.7 15.1 13.8 10.2 4.0    
Reinsurance | Operating Segments | Marine & Energy | 2015                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 27.5 27.5 27.1 27.3 27.8 30.0 27.8      
IBNR loss and ALAE reserves, net (0.1)                  
Cumulative net losses and allocated loss adjustment expenses paid 26.6 26.5 25.8 24.7 20.3 10.4 3.1      
Reinsurance | Operating Segments | Marine & Energy | 2016                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 25.0 25.1 25.0 25.2 28.9 30.5        
IBNR loss and ALAE reserves, net 0.1                  
Cumulative net losses and allocated loss adjustment expenses paid 23.6 22.6 20.6 18.0 14.6 6.1        
Reinsurance | Operating Segments | Marine & Energy | 2017                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 31.9 32.3 29.9 30.8 35.0          
IBNR loss and ALAE reserves, net 1.3                  
Cumulative net losses and allocated loss adjustment expenses paid 25.7 24.0 19.7 13.4 5.0          
Reinsurance | Operating Segments | Marine & Energy | 2018                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 20.9 21.1 21.2 19.5            
IBNR loss and ALAE reserves, net 1.4                  
Cumulative net losses and allocated loss adjustment expenses paid 15.8 15.0 9.2 3.0            
Reinsurance | Operating Segments | Marine & Energy | 2019                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 19.7 19.4 19.0              
IBNR loss and ALAE reserves, net 3.9                  
Cumulative net losses and allocated loss adjustment expenses paid 10.6 7.6 2.6              
Reinsurance | Operating Segments | Marine & Energy | 2020                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 18.6 20.5                
IBNR loss and ALAE reserves, net 6.2                  
Cumulative net losses and allocated loss adjustment expenses paid 6.6 2.1                
Reinsurance | Operating Segments | Marine & Energy | 2021                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 25.2                  
IBNR loss and ALAE reserves, net 16.8                  
Cumulative net losses and allocated loss adjustment expenses paid 2.0                  
Reinsurance | Operating Segments | Mortgage                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 41.8                  
IBNR loss and ALAE reserves, net 17.0                  
Cumulative net losses and allocated loss adjustment expenses paid 9.3                  
Net reserves for loss and allocated loss adjustment expenses from 2012 to 2021 32.5                  
Net reserves for loss and allocated loss adjustment expenses prior to 2012 (0.1)                  
Net reserves for loss and allocated loss adjustment expenses, end of year 32.4                  
Reinsurance | Operating Segments | Mortgage | 2012                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
IBNR loss and ALAE reserves, net 0.0                  
Cumulative net losses and allocated loss adjustment expenses paid 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
Reinsurance | Operating Segments | Mortgage | 2013                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.3 0.3  
IBNR loss and ALAE reserves, net 0.0                  
Cumulative net losses and allocated loss adjustment expenses paid 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.0 0.0  
Reinsurance | Operating Segments | Mortgage | 2014                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 0.6 0.6 0.6 0.6 0.6 0.6 0.6 3.5    
IBNR loss and ALAE reserves, net 0.0                  
Cumulative net losses and allocated loss adjustment expenses paid 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.0    
Reinsurance | Operating Segments | Mortgage | 2015                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 0.8 0.8 0.8 1.7 1.6 1.6 1.6      
IBNR loss and ALAE reserves, net 0.0                  
Cumulative net losses and allocated loss adjustment expenses paid 0.8 0.8 0.8 0.7 0.7 0.5 0.3      
Reinsurance | Operating Segments | Mortgage | 2016                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 1.9 2.2 1.4 5.8 5.5 5.8        
IBNR loss and ALAE reserves, net 0.0                  
Cumulative net losses and allocated loss adjustment expenses paid 1.0 1.0 1.0 0.8 0.5 0.1        
Reinsurance | Operating Segments | Mortgage | 2017                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 2.6 3.0 2.1 8.3 7.9          
IBNR loss and ALAE reserves, net 0.5                  
Cumulative net losses and allocated loss adjustment expenses paid 1.0 1.1 1.0 0.7 0.1          
Reinsurance | Operating Segments | Mortgage | 2018                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 4.9 5.4 4.2 11.1            
IBNR loss and ALAE reserves, net 1.3                  
Cumulative net losses and allocated loss adjustment expenses paid 1.4 1.3 1.2 0.3            
Reinsurance | Operating Segments | Mortgage | 2019                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 8.0 8.7 6.9              
IBNR loss and ALAE reserves, net 2.5                  
Cumulative net losses and allocated loss adjustment expenses paid 1.8 1.6 0.8              
Reinsurance | Operating Segments | Mortgage | 2020                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 11.8 11.9                
IBNR loss and ALAE reserves, net 5.8                  
Cumulative net losses and allocated loss adjustment expenses paid 1.5 1.4                
Reinsurance | Operating Segments | Mortgage | 2021                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 11.1                  
IBNR loss and ALAE reserves, net 6.9                  
Cumulative net losses and allocated loss adjustment expenses paid 1.1                  
Reinsurance | Operating Segments | Property                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 3,995.6                  
IBNR loss and ALAE reserves, net 558.0                  
Cumulative net losses and allocated loss adjustment expenses paid 2,832.9                  
Net reserves for loss and allocated loss adjustment expenses from 2012 to 2021 1,162.7                  
Net reserves for loss and allocated loss adjustment expenses prior to 2012 33.0                  
Net reserves for loss and allocated loss adjustment expenses, end of year 1,195.7                  
Reinsurance | Operating Segments | Property | 2012                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 276.0 276.9 276.2 275.4 275.2 280.2 283.0 289.8 300.8 314.1
IBNR loss and ALAE reserves, net 1.1                  
Cumulative net losses and allocated loss adjustment expenses paid 270.5 268.9 268.4 265.2 263.2 259.1 253.5 241.1 207.7 46.6
Reinsurance | Operating Segments | Property | 2013                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 239.5 240.2 239.8 240.5 240.8 242.2 245.5 256.7 232.6  
IBNR loss and ALAE reserves, net 0.6                  
Cumulative net losses and allocated loss adjustment expenses paid 234.3 234.3 233.0 231.3 229.5 224.8 212.0 171.6 56.7  
Reinsurance | Operating Segments | Property | 2014                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 213.4 213.1 213.6 213.9 213.7 209.4 211.6 206.7    
IBNR loss and ALAE reserves, net 1.7                  
Cumulative net losses and allocated loss adjustment expenses paid 206.9 206.0 205.1 201.0 194.5 177.1 141.5 50.4    
Reinsurance | Operating Segments | Property | 2015                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 217.5 217.3 217.5 218.9 218.4 214.6 217.2      
IBNR loss and ALAE reserves, net 2.5                  
Cumulative net losses and allocated loss adjustment expenses paid 211.0 208.9 206.6 198.0 181.7 147.2 53.1      
Reinsurance | Operating Segments | Property | 2016                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 294.2 295.0 297.1 296.9 295.0 294.3        
IBNR loss and ALAE reserves, net 3.3                  
Cumulative net losses and allocated loss adjustment expenses paid 284.2 280.2 268.8 244.9 191.7 63.8        
Reinsurance | Operating Segments | Property | 2017                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 521.5 521.9 516.2 506.2 462.9          
IBNR loss and ALAE reserves, net 19.0                  
Cumulative net losses and allocated loss adjustment expenses paid 472.7 460.3 404.3 315.9 88.0          
Reinsurance | Operating Segments | Property | 2018                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 535.8 542.8 535.5 474.7            
IBNR loss and ALAE reserves, net 18.5                  
Cumulative net losses and allocated loss adjustment expenses paid 461.1 428.4 330.7 69.3            
Reinsurance | Operating Segments | Property | 2019                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 525.1 526.8 542.5              
IBNR loss and ALAE reserves, net 37.7                  
Cumulative net losses and allocated loss adjustment expenses paid 400.4 305.8 68.4              
Reinsurance | Operating Segments | Property | 2020                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 569.9 577.4                
IBNR loss and ALAE reserves, net 188.3                  
Cumulative net losses and allocated loss adjustment expenses paid 225.5 77.7                
Reinsurance | Operating Segments | Property | 2021                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 602.7                  
IBNR loss and ALAE reserves, net 285.3                  
Cumulative net losses and allocated loss adjustment expenses paid 66.3                  
Insurance & Services | Operating Segments | A&H                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 1,873.7                  
IBNR loss and ALAE reserves, net 143.2                  
Cumulative net losses and allocated loss adjustment expenses paid 1,696.4                  
Net reserves for loss and allocated loss adjustment expenses from 2012 to 2021 177.3                  
Net reserves for loss and allocated loss adjustment expenses prior to 2012 (0.5)                  
Net reserves for loss and allocated loss adjustment expenses, end of year 176.8                  
Insurance & Services | Operating Segments | A&H | 2012                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 147.3 147.3 147.3 147.5 147.6 147.7 148.0 148.7 161.0 164.2
IBNR loss and ALAE reserves, net (0.1)                  
Cumulative net losses and allocated loss adjustment expenses paid 147.5 147.5 147.5 147.5 147.5 147.5 147.3 147.0 136.9 72.5
Insurance & Services | Operating Segments | A&H | 2013                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 117.5 117.4 117.6 117.7 118.5 119.2 119.7 124.2 126.3  
IBNR loss and ALAE reserves, net 0.0                  
Cumulative net losses and allocated loss adjustment expenses paid 117.1 117.1 117.1 117.1 117.7 116.4 115.0 104.5 54.7  
Insurance & Services | Operating Segments | A&H | 2014                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 129.9 129.8 129.9 129.9 130.8 130.9 132.5 131.4    
IBNR loss and ALAE reserves, net 0.1                  
Cumulative net losses and allocated loss adjustment expenses paid 127.7 127.6 127.5 126.8 126.8 125.1 111.6 59.4    
Insurance & Services | Operating Segments | A&H | 2015                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 144.7 144.6 144.6 145.0 146.3 149.7 153.9      
IBNR loss and ALAE reserves, net 0.3                  
Cumulative net losses and allocated loss adjustment expenses paid 144.0 144.0 143.9 143.6 141.9 130.4 75.8      
Insurance & Services | Operating Segments | A&H | 2016                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 168.1 168.6 168.8 170.3 174.4 174.8        
IBNR loss and ALAE reserves, net 2.3                  
Cumulative net losses and allocated loss adjustment expenses paid 165.4 165.7 165.2 163.3 151.5 99.0        
Insurance & Services | Operating Segments | A&H | 2017                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 165.5 165.7 168.0 174.6 178.4          
IBNR loss and ALAE reserves, net (0.5)                  
Cumulative net losses and allocated loss adjustment expenses paid 163.8 163.5 162.6 149.8 58.8          
Insurance & Services | Operating Segments | A&H | 2018                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 205.9 207.7 209.9 202.3            
IBNR loss and ALAE reserves, net (1.2)                  
Cumulative net losses and allocated loss adjustment expenses paid 207.4 206.3 189.4 89.4            
Insurance & Services | Operating Segments | A&H | 2019                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 263.2 272.1 277.1              
IBNR loss and ALAE reserves, net 0.8                  
Cumulative net losses and allocated loss adjustment expenses paid 254.6 237.6 129.8              
Insurance & Services | Operating Segments | A&H | 2020                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 307.3 313.2                
IBNR loss and ALAE reserves, net 56.4                  
Cumulative net losses and allocated loss adjustment expenses paid 247.7 105.4                
Insurance & Services | Operating Segments | A&H | 2021                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 224.3                  
IBNR loss and ALAE reserves, net 85.1                  
Cumulative net losses and allocated loss adjustment expenses paid 121.2                  
Insurance & Services | Operating Segments | Environmental                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 10.7                  
IBNR loss and ALAE reserves, net 4.5                  
Cumulative net losses and allocated loss adjustment expenses paid 2.2                  
Net reserves for loss and allocated loss adjustment expenses from 2012 to 2021 8.5                  
Net reserves for loss and allocated loss adjustment expenses prior to 2012 0.0                  
Net reserves for loss and allocated loss adjustment expenses, end of year 8.5                  
Insurance & Services | Operating Segments | Environmental | 2012                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
IBNR loss and ALAE reserves, net 0.0                  
Cumulative net losses and allocated loss adjustment expenses paid 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
Insurance & Services | Operating Segments | Environmental | 2013                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0  
IBNR loss and ALAE reserves, net 0.0                  
Cumulative net losses and allocated loss adjustment expenses paid 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0  
Insurance & Services | Operating Segments | Environmental | 2014                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0    
IBNR loss and ALAE reserves, net 0.0                  
Cumulative net losses and allocated loss adjustment expenses paid 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0    
Insurance & Services | Operating Segments | Environmental | 2015                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 0.0 0.0 0.0 0.0 0.0 0.0 0.0      
IBNR loss and ALAE reserves, net 0.0                  
Cumulative net losses and allocated loss adjustment expenses paid 0.0 0.0 0.0 0.0 0.0 0.0 0.0      
Insurance & Services | Operating Segments | Environmental | 2016                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 0.0 0.0 0.0 0.0 0.0 0.0        
IBNR loss and ALAE reserves, net 0.0                  
Cumulative net losses and allocated loss adjustment expenses paid 0.0 0.0 0.0 0.0 0.0 0.0        
Insurance & Services | Operating Segments | Environmental | 2017                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 0.0 0.0 0.0 0.0 0.0          
IBNR loss and ALAE reserves, net 0.0                  
Cumulative net losses and allocated loss adjustment expenses paid 0.0 0.0 0.0 0.0 0.0          
Insurance & Services | Operating Segments | Environmental | 2018                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 0.1 0.1 0.1 0.4            
IBNR loss and ALAE reserves, net 0.0                  
Cumulative net losses and allocated loss adjustment expenses paid 0.1 0.1 0.0 0.0            
Insurance & Services | Operating Segments | Environmental | 2019                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 2.7 4.6 4.5              
IBNR loss and ALAE reserves, net (1.9)                  
Cumulative net losses and allocated loss adjustment expenses paid 1.8 0.9 0.0              
Insurance & Services | Operating Segments | Environmental | 2020                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 3.2 3.6                
IBNR loss and ALAE reserves, net 2.8                  
Cumulative net losses and allocated loss adjustment expenses paid 0.3 0.0                
Insurance & Services | Operating Segments | Environmental | 2021                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 4.7                  
IBNR loss and ALAE reserves, net 3.6                  
Cumulative net losses and allocated loss adjustment expenses paid 0.0                  
Insurance & Services | Operating Segments | Workers’ Compensation                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 158.7                  
IBNR loss and ALAE reserves, net 76.0                  
Cumulative net losses and allocated loss adjustment expenses paid 40.5                  
Net reserves for loss and allocated loss adjustment expenses from 2012 to 2021 118.2                  
Net reserves for loss and allocated loss adjustment expenses prior to 2012 0.0                  
Net reserves for loss and allocated loss adjustment expenses, end of year 118.2                  
Insurance & Services | Operating Segments | Workers’ Compensation | 2012                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
IBNR loss and ALAE reserves, net 0.0                  
Cumulative net losses and allocated loss adjustment expenses paid 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
Insurance & Services | Operating Segments | Workers’ Compensation | 2013                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0  
IBNR loss and ALAE reserves, net 0.0                  
Cumulative net losses and allocated loss adjustment expenses paid 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0  
Insurance & Services | Operating Segments | Workers’ Compensation | 2014                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0    
IBNR loss and ALAE reserves, net 0.0                  
Cumulative net losses and allocated loss adjustment expenses paid 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0    
Insurance & Services | Operating Segments | Workers’ Compensation | 2015                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 0.0 0.0 0.0 0.0 0.0 0.0 0.0      
IBNR loss and ALAE reserves, net 0.0                  
Cumulative net losses and allocated loss adjustment expenses paid 0.0 0.0 0.0 0.0 0.0 0.0 0.0      
Insurance & Services | Operating Segments | Workers’ Compensation | 2016                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 0.0 0.0 0.0 0.0 0.0 0.0        
IBNR loss and ALAE reserves, net 0.0                  
Cumulative net losses and allocated loss adjustment expenses paid 0.0 0.0 0.0 0.0 0.0 0.0        
Insurance & Services | Operating Segments | Workers’ Compensation | 2017                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 0.0 0.0 0.0 0.0 0.0          
IBNR loss and ALAE reserves, net 0.0                  
Cumulative net losses and allocated loss adjustment expenses paid 0.0 0.0 0.0 0.0 0.0          
Insurance & Services | Operating Segments | Workers’ Compensation | 2018                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 1.2 1.1 1.5 1.5            
IBNR loss and ALAE reserves, net 0.6                  
Cumulative net losses and allocated loss adjustment expenses paid 0.4 0.3 0.2 0.0            
Insurance & Services | Operating Segments | Workers’ Compensation | 2019                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 15.7 16.6 18.6              
IBNR loss and ALAE reserves, net 2.8                  
Cumulative net losses and allocated loss adjustment expenses paid 10.0 6.8 1.3              
Insurance & Services | Operating Segments | Workers’ Compensation | 2020                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 46.9 45.8                
IBNR loss and ALAE reserves, net 11.3                  
Cumulative net losses and allocated loss adjustment expenses paid 19.7 4.2                
Insurance & Services | Operating Segments | Workers’ Compensation | 2021                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 94.9                  
IBNR loss and ALAE reserves, net 61.3                  
Cumulative net losses and allocated loss adjustment expenses paid 10.4                  
Insurance & Services | Operating Segments | Other                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 64.8                  
IBNR loss and ALAE reserves, net 61.3                  
Cumulative net losses and allocated loss adjustment expenses paid 2.3                  
Net reserves for loss and allocated loss adjustment expenses from 2012 to 2021 62.5                  
Net reserves for loss and allocated loss adjustment expenses prior to 2012 0.0                  
Net reserves for loss and allocated loss adjustment expenses, end of year 62.5                  
Insurance & Services | Operating Segments | Other | 2012                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
IBNR loss and ALAE reserves, net 0.0                  
Cumulative net losses and allocated loss adjustment expenses paid 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 $ 0.0
Insurance & Services | Operating Segments | Other | 2013                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0  
IBNR loss and ALAE reserves, net 0.0                  
Cumulative net losses and allocated loss adjustment expenses paid 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 $ 0.0  
Insurance & Services | Operating Segments | Other | 2014                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0    
IBNR loss and ALAE reserves, net 0.0                  
Cumulative net losses and allocated loss adjustment expenses paid 0.0 0.0 0.0 0.0 0.0 0.0 0.0 $ 0.0    
Insurance & Services | Operating Segments | Other | 2015                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 0.0 0.0 0.0 0.0 0.0 0.0 0.0      
IBNR loss and ALAE reserves, net 0.0                  
Cumulative net losses and allocated loss adjustment expenses paid 0.0 0.0 0.0 0.0 0.0 0.0 $ 0.0      
Insurance & Services | Operating Segments | Other | 2016                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 0.0 0.0 0.0 0.0 0.0 0.0        
IBNR loss and ALAE reserves, net 0.0                  
Cumulative net losses and allocated loss adjustment expenses paid 0.0 0.0 0.0 0.0 0.0 $ 0.0        
Insurance & Services | Operating Segments | Other | 2017                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 0.0 0.0 0.0 0.0 0.0          
IBNR loss and ALAE reserves, net 0.0                  
Cumulative net losses and allocated loss adjustment expenses paid 0.0 0.0 0.0 0.0 $ 0.0          
Insurance & Services | Operating Segments | Other | 2018                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 0.0 0.0 0.0 0.0            
IBNR loss and ALAE reserves, net 0.0                  
Cumulative net losses and allocated loss adjustment expenses paid 0.0 0.0 0.0 $ 0.0            
Insurance & Services | Operating Segments | Other | 2019                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 0.0 0.0 0.0              
IBNR loss and ALAE reserves, net 0.0                  
Cumulative net losses and allocated loss adjustment expenses paid 0.0 0.0 $ 0.0              
Insurance & Services | Operating Segments | Other | 2020                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 2.3 2.6                
IBNR loss and ALAE reserves, net 1.4                  
Cumulative net losses and allocated loss adjustment expenses paid 0.8 $ 0.4                
Insurance & Services | Operating Segments | Other | 2021                    
Claims Development [Line Items]                    
Loss and allocated loss adjustment expenses incurred, net 62.5                  
IBNR loss and ALAE reserves, net 59.9                  
Cumulative net losses and allocated loss adjustment expenses paid $ 1.5