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Reserves for Losses and Adjustment Expenses Short-duration insurance contracts - Reconciliation of Claims Development (Details) - USD ($)
$ in Millions
Sep. 30, 2024
Dec. 31, 2023
Sep. 30, 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)    
Losses and loss adjustment expenses $ 8,201.3 7,810.6 $ 7,770.8 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)    
Losses and loss adjustment expenses   $ 27.3   $ 42.7