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Reserves for Losses and Adjustment Expenses Short-duration insurance contracts - Reconciliation of Claims Development (Details) - USD ($)
$ in Millions
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for claims and claim adjustment expenses, net of reinsurance $ 3,629.6 $ 3,151.6    
Reinsurance recoverable on unpaid losses 4,577.8 4,897.7    
Unallocated claims incurred 47.9 41.4    
Carbon syndicate reserves 16.7 5.2    
Other 0.4 1.6    
Liability for unpaid claims and claim adjustment expense, aggregate reconciling items (396.8) (338.4)    
Reserve for losses and loss adjustment expenses 7,810.6 7,710.9 $ 7,611.8 $ 7,165.3
Property Insurance        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for claims and claim adjustment expenses, net of reinsurance 270.8 259.0    
Casualty Insurance        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for claims and claim adjustment expenses, net of reinsurance 636.5 457.3    
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 194.5 163.7    
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 837.6 671.9    
Insurance lines        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for claims and claim adjustment expenses, net of reinsurance 1,939.4 1,551.9    
Reinsurance recoverable on unpaid losses 2,821.6 2,907.8    
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 562.3 677.9    
Casualty Reinsurance        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for claims and claim adjustment expenses, net of reinsurance 733.6 571.6    
Specialty Reinsurance        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for claims and claim adjustment expenses, net of reinsurance 394.3 350.2    
Reinsurance lines        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for claims and claim adjustment expenses, net of reinsurance 1,690.2 1,599.7    
Reinsurance recoverable on unpaid losses 1,756.2 1,989.9    
Insurance lines other than short-duration        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Other reinsurance balances recoverable (489.1) (429.3)    
Reserve for losses and loss adjustment expenses $ 27.3 $ 42.7