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Reserve for Losses and Loss Adjustment Expenses - Narrative (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2023
Sep. 30, 2022
Dec. 31, 2022
Dec. 31, 2021
Liability for Claims and Claims Adjustment Expense [Line Items]            
Net prior year development, unfavorable (favorable)     $ (8,903) $ (3,522)    
Acquisition costs $ 78,537 $ 65,735 220,532 194,795    
Reserve for losses and loss adjustment expenses 2,948,822 $ 2,756,197 2,948,822 2,756,197 $ 2,856,275 $ 2,415,491
Ukrainian Conflict            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Reserve for losses and loss adjustment expenses 66,200   66,200   79,300  
Covid-19            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Reserve for losses and loss adjustment expenses $ 13,900   13,900   $ 39,000  
Specialty Contracts            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Net prior year development, unfavorable (favorable)     (20,300) (23,600)    
Casualty Lines            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Net prior year development, unfavorable (favorable)     8,800 27,700    
Casualty Lines | Reinsurance Contract [Axis]: Loss Portfolio Transfer Agreement            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Net prior year development, unfavorable (favorable)     (4,000) (6,000)    
Property Contracts            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Net prior year development, unfavorable (favorable)     6,600 (1,600)    
Attritional Losses            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Net prior year development, unfavorable (favorable)     (4,000) 11,500    
Catastrophe Losses            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Net prior year development, unfavorable (favorable)     $ (4,900) $ (15,000)