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Reserve for Losses and Loss Adjustment Expenses - Narrative (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2024
Sep. 30, 2023
Sep. 30, 2024
Sep. 30, 2023
Dec. 31, 2023
Dec. 31, 2022
Liability for Claims and Claims Adjustment Expense [Line Items]            
Net prior year development, unfavorable (favorable)     $ (6,124) $ (8,903)    
Acquisition costs $ 102,201 $ 78,537 283,059 220,532    
Reserve for losses and loss adjustment expenses 3,434,800 2,948,822 3,434,800 2,948,822 $ 3,030,037 $ 2,856,275
Hurricane Helene            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Reserve for losses and loss adjustment expenses 33,900   33,900   0  
Baltimore Bridge            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Reserve for losses and loss adjustment expenses 37,900   37,900   0  
Ukrainian Conflict            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Reserve for losses and loss adjustment expenses 63,700   63,700   64,900  
Covid-19            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Reserve for losses and loss adjustment expenses 15,600   15,600   $ 14,100  
Specialty Contracts            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Net prior year development, unfavorable (favorable)     9,500 (20,300)    
Casualty Lines            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Net prior year development, unfavorable (favorable)     13,200 8,800    
Property Contracts            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Net prior year development, unfavorable (favorable)     (26,100) 6,600    
Casualty Lines of Business | Reinsurance Contract [Axis]: Loss Portfolio Transfer Agreement            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Net prior year development, unfavorable (favorable)     (2,700) (4,000)    
Amortization of the associated deferred gain     (1,300) (2,500)    
Net unfavorable (favorable) development in the underlying reserves $ (1,400) $ (1,500) (1,400) (1,500)    
Attritional Losses            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Net prior year development, unfavorable (favorable)     7,100 (4,000)    
Catastrophe Losses            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Net prior year development, unfavorable (favorable)     $ (13,200) $ (4,900)