v3.26.1
Reserve for Losses and Loss Adjustment Expenses - Narrative (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Dec. 31, 2025
Dec. 31, 2024
Liability for Claims and Claims Adjustment Expense [Line Items]            
Net prior year development, unfavorable (favorable)     $ 17,276 $ (34,692)    
Acquisition costs $ 145,423 $ 122,815 289,929 239,696    
Reserve for losses and loss adjustment expenses 4,783,094 3,984,281 4,783,094 3,984,281 $ 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 11,400   11,400   20,500  
Middle East Conflict            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Reserve for losses and loss adjustment expenses 45,500   45,500   0  
California Wildfires            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Reserve for losses and loss adjustment expenses 45,900   45,900   57,500  
Ukrainian Conflict            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Reserve for losses and loss adjustment expenses 8,000   8,000   $ 59,500  
Casualty Lines            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Net prior year development, unfavorable (favorable)     15,900 20,400    
Casualty Lines | Reinsurance Contract [Axis]: Loss Portfolio Transfer Agreement            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Net prior year development, unfavorable (favorable)     (200) (1,200)    
Net unfavorable (favorable) development in the underlying reserves $ 2,400 $ (3,900) 2,400 (3,900)    
Amortization of the associated deferred gain     (2,600) 2,700    
Specialty Contracts            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Net prior year development, unfavorable (favorable)     (12,300) (18,300)    
Property Contracts            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Net prior year development, unfavorable (favorable)     (600) (35,600)    
Attritional Losses            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Net prior year development, unfavorable (favorable)     13,100 (17,100)    
Catastrophe Losses            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Net prior year development, unfavorable (favorable)     $ 4,200 $ (17,600)