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Reserve for Loss and Loss Expense (Rollforward of Reserve for Loss and Loss Expense) (Details) - USD ($)
$ in Thousands
 6 Months Ended
Jun. 30, 2022
Jun. 30, 2021
Mar. 31, 2022
Dec. 31, 2021
Mar. 31, 2021
Dec. 31, 2020
Liability for Claims and Claims Adjustment Expense [Line Items]            
Gross reserve for loss and loss expense, at beginning of period $ 4,580,903 $ 4,260,355        
Less: reinsurance recoverable on unpaid loss and loss expense, at beginning of period 578,641 554,269        
Net reserve for loss and loss expense, at beginning of period 4,002,262 3,706,086        
Incurred loss and loss expense for claims occurring in the current year 1,041,778 886,801        
Total incurred loss and loss expense 1,019,104 835,024        
Paid loss and loss expense for claims occurring in the current year 272,401 227,505        
Paid loss and loss expense for claims occurring in the prior years 570,868 455,919        
Total paid loss and loss expense 843,269 683,424        
Net reserve for loss and loss expense, at end of period 4,178,097 3,857,686        
Add: reinsurance recoverable on unpaid loss and loss expense, at end of period 544,082 579,567        
Gross reserve for loss and loss expense, at end of period 4,722,179 4,437,253        
Less: Allowance for credit losses (Note 7) (1,600) (1,777) $ (1,600) $ (1,600) $ (1,840) $ (1,777)
Change in Historical Claim Rate Experience [Member]            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Prior year development (22,674) (51,777)        
Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member]            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Prior year development 32,000 52,000        
Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member] | Workers' Compensation Insurance [Member]            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Prior year development 20,000 20,000        
Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member] | General Liability [Member]            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Prior year development 5,000 25,000        
Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member] | E&S Lines            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Prior year development   $ 7,000        
Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member] | Surety Lines            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Prior year development 7,000          
Property Insurance [Member] | Change in Historical Claim Rate Experience [Member]            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Prior year development $ 9,300