v3.26.1
Reserves for Losses and Adjustment Expenses Short-duration insurance contracts - Reconciliation of Claims Development (Details) - USD ($)
$ in Millions
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for claims and claim adjustment expenses, net of reinsurance $ 4,492.4 $ 4,116.6    
Reinsurance recoverable on unpaid losses 4,281.9 4,172.0    
Unallocated claims incurred 71.8 58.0    
Carbon syndicate reserves 55.3 26.5    
Liability for unpaid claims and claim adjustment expense, aggregate reconciling items (49.3) (166.0)    
Liability for Claims and Claims Adjustment Expense 8,725.0 [1] 8,122.6 [1] $ 7,810.6 $ 7,710.9
Property Insurance        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for claims and claim adjustment expenses, net of reinsurance 211.1 268.9    
Casualty Insurance        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for claims and claim adjustment expenses, net of reinsurance 898.3 756.0    
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 220.1 221.3    
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 1,109.1 981.3    
Insurance lines        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for claims and claim adjustment expenses, net of reinsurance 2,438.6 2,227.5    
Reinsurance recoverable on unpaid losses 2,852.3 2,698.0    
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 487.3 560.6    
Casualty Reinsurance        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for claims and claim adjustment expenses, net of reinsurance 1,064.0 885.9    
Specialty Reinsurance        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for claims and claim adjustment expenses, net of reinsurance 502.5 442.6    
Reinsurance lines        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Liability for claims and claim adjustment expenses, net of reinsurance 2,053.8 1,889.1    
Reinsurance recoverable on unpaid losses 1,429.6 1,474.0    
Insurance lines other than short-duration        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Other reinsurance balances recoverable (259.9) (311.7)    
Liability for Claims and Claims Adjustment Expense $ 83.5 $ 61.2    
[1] Included within underwriting premiums receivables, reserve for losses and loss adjustment expenses and unearned premiums are related party balances of $1.6 million, $3.7 million and $1.2 million, respectively. Refer to Note 20, “Related Party Transactions” for further details.