v3.26.3
Reserve for Losses and Loss Adjustment Expenses - Narrative (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Dec. 31, 2025
Dec. 31, 2024
Liability for Claims and Claims Adjustment Expense [Line Items]            
Net prior year development, unfavorable (favorable)     $ 37,119 $ (45,854)    
Acquisition costs $ 150,972 $ 125,412 440,902 365,108    
Reserve for losses and loss adjustment expenses 4,988,921 4,206,077 4,988,921 4,206,077 $ 4,415,176 $ 3,532,491
Baltimore Bridge            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Net prior year development, unfavorable (favorable)     14,500      
Reserve for losses and loss adjustment expenses 11,800   11,800   20,500  
Middle East Conflict            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Reserve for losses and loss adjustment expenses 41,700   41,700   0  
California Wildfires            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Reserve for losses and loss adjustment expenses 43,100   43,100   57,500  
Ukrainian Conflict            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Net prior year development, unfavorable (favorable)     47,100      
Reserve for losses and loss adjustment expenses 74,800   74,800   $ 59,500  
Casualty Lines            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Net prior year development, unfavorable (favorable)     17,200 19,900    
Casualty Lines | Reinsurance Contract [Axis]: Loss Portfolio Transfer Agreement            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Net prior year development, unfavorable (favorable)     (400) (1,700)    
Net unfavorable (favorable) development in the underlying reserves $ 3,000 $ (4,200) 3,000 (4,200)    
Amortization of the associated deferred gain     (3,400) 2,500    
Property Contracts            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Net prior year development, unfavorable (favorable)     500 (42,000)    
Specialty Contracts            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Net prior year development, unfavorable (favorable)     (41,800) (22,100)    
Attritional Losses            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Net prior year development, unfavorable (favorable)     (14,100) (28,300)    
Catastrophe Losses            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Net prior year development, unfavorable (favorable)     $ 51,200 $ (17,600)