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Reserve for Loss and Loss Expense (Rollforward of Reserve for Loss and Loss Expense) (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2022
Mar. 31, 2021
Dec. 31, 2021
Dec. 31, 2020
Liability for Claims and Claims Adjustment Expense [Line Items]        
Gross reserve for loss and loss expense, at beginning of year $ 4,580,903 $ 4,260,355    
Less: reinsurance recoverable on unpaid loss and loss expense, at beginning of year 578,641 554,269    
Net reserve for loss and loss expense, at beginning of year 4,002,262 3,706,086    
Incurred loss and loss expense for claims occurring in the current year 508,299 447,170    
Total incurred loss and loss expense 494,236 413,401    
Paid loss and loss expense for claims occurring in the current year 91,292 80,158    
Paid loss and loss expense for claims occurring in the prior years 312,926 243,687    
Total paid loss and loss expense 404,218 323,845    
Net reserve for loss and loss expense, at end of period 4,092,280 3,795,642    
Add: reinsurance recoverable on unpaid loss and loss expense, at end of period 552,111 564,546    
Gross reserve for loss and loss expense, at end of period 4,644,391 4,360,188    
Allowance for credit losses (1,600) (1,840) $ (1,600) $ (1,777)
Change in Historical Claim Rate Experience [Member]        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Prior year development (14,063) (33,769)    
Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member]        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Prior year development 20,000 35,000    
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 10,000 15,000    
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 15,000    
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   5,000    
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 5,000      
Property Insurance [Member] | Change in Historical Claim Rate Experience [Member]        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Prior year development $ 5,900 $ 1,200