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Reserves for Losses and Adjustment Expenses Short-duration insurance contracts - Reconciliation of Claims Development (Details) - USD ($)
$ in Millions
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for claims and claim adjustment expenses, net of reinsurance $ 4,484.0 $ 4,266.0    
Reinsurance recoverable on unpaid losses 3,298.1 3,195.2    
Unallocated claims incurred 56.6 50.3    
Carbon syndicate reserves 1.0 0.0    
Other 0.2 0.5    
Liability for unpaid claims and claim adjustment expense, aggregate reconciling items (170.3) (295.9)    
Losses and loss adjustment expenses 7,611.8 7,165.3 $ 6,951.8 $ 7,074.2
Property Insurance        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for claims and claim adjustment expenses, net of reinsurance 289.3 226.8    
Casualty Insurance        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for claims and claim adjustment expenses, net of reinsurance 725.6 618.7    
Marine, Aviation and Energy Insurance        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for claims and claim adjustment expenses, net of reinsurance 427.9 431.4    
Financial and Professional Insurance        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for claims and claim adjustment expenses, net of reinsurance 808.7 738.5    
Insurance lines        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for claims and claim adjustment expenses, net of reinsurance 2,251.5 2,015.4    
Reinsurance recoverable on unpaid losses 2,117.4 2,140.3    
Property Catastrophe and Other Property Reinsurance        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for claims and claim adjustment expenses, net of reinsurance 784.9 631.0    
Casualty Reinsurance        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for claims and claim adjustment expenses, net of reinsurance 975.3 1,084.2    
Specialty Reinsurance        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for claims and claim adjustment expenses, net of reinsurance 472.3 535.4    
Reinsurance lines        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for claims and claim adjustment expenses, net of reinsurance 2,232.5 2,250.6    
Reinsurance recoverable on unpaid losses 1,180.7 1,054.9    
Insurance lines other than short-duration        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Other reinsurance balances recoverable (286.4) (346.7)    
Losses and loss adjustment expenses $ 58.3 $ 0.0