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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, 2021
Mar. 31, 2020
Dec. 31, 2020
Dec. 31, 2019
Liability for Claims and Claims Adjustment Expense [Line Items]        
Gross reserve for loss and loss expense, at beginning of year $ 4,260,355 $ 4,067,163    
Less: reinsurance recoverable on unpaid loss and loss expense, at beginning of year 554,269 547,066 [1]    
Net reserve for loss and loss expense, at beginning of year 3,706,086 3,520,097    
Incurred loss and loss expense for claims occurring in the current year 447,170 407,276    
Total incurred loss and loss expense 413,401 400,324    
Paid loss and loss expense for claims occurring in the current year 80,158 70,610    
Paid loss and loss expense for claims occurring in the prior years 243,687 281,736    
Total paid loss and loss expense 323,845 352,346    
Net reserve for loss and loss expense, at end of period 3,795,642 3,568,075    
Add: reinsurance recoverable on unpaid loss and loss expense, at end of period 564,546 535,560    
Gross reserve for loss and loss expense, at end of period 4,360,188 4,103,635    
Allowance for credit losses (1,840) (1,502) $ (1,777) $ (4,400)
Change in Historical Claim Rate Experience [Member]        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Prior year development (33,769) (6,952)    
Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member]        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Prior year development (35,000) (10,000)    
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 (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 (15,000)      
Property Insurance [Member] | Change in Historical Claim Rate Experience [Member]        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Prior year development 1,200 $ 3,000    
E&S Lines | Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member]        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Prior year development $ (5,000)      
Reinsurance Recoverable [Domain]        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Allowance for credit losses     (1,777) (1,497)
Reinsurance Recoverable [Domain] | Retained Earnings [Member] | Accounting Standards Update 2016-13 [Member]        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Allowance for credit losses     $ 0 $ 2,903 [1]
[1] includes an adjustment of $2.9 million related to our adoption of ASU 2016-13, Financial Instruments - Credit Losses.