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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, 2020
Jun. 30, 2019
Mar. 31, 2020
Dec. 31, 2019
Mar. 31, 2019
Dec. 31, 2018
Liability for Claims and Claims Adjustment Expense [Line Items]            
Gross reserve for loss and loss expense, at beginning of year $ 4,067,163 $ 3,893,868        
Less: reinsurance recoverable on unpaid loss and loss expense, at beginning of year 547,066 [1] 537,388        
Net reserve for loss and loss expense, at beginning of year 3,520,097 3,356,480        
Incurred loss and loss expense for claims occurring in the current year 825,201 783,938        
Total incurred loss and loss expense 804,273 767,563        
Paid loss and loss expense for claims occurring in the current year 221,422 210,374        
Paid loss and loss expense for claims occurring in the prior years 479,736 442,820        
Total paid loss and loss expense 701,158 653,194        
Net reserve for loss and loss expense, at end of period 3,623,212 3,470,849        
Add: reinsurance recoverable on unpaid loss and loss expense, at end of period 553,694 556,203        
Gross reserve for loss and loss expense, at end of period 4,176,906 4,027,052        
Loss Reserve for COVID-19 10,000          
Loss from Catastrophes 116,400 50,300        
Change in Historical Claim Rate Experience [Member]            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Prior year development (20,928) (16,375)        
Workers' Compensation Insurance [Member] | Change in Historical Claim Rate Experience [Member]            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Prior year development (25,000)          
Property Insurance [Member] | Change in Historical Claim Rate Experience [Member]            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Prior year development 4,100 10,600        
Workers' Compensation Insurance [Member] | Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member]            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Prior year development (25,000) (20,000)        
General Liability [Member] | Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member]            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Prior year development (10,000) $ (7,000)        
Commercial Automobile [Member] | Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member]            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Prior year development $ 10,000          
Accounting Standards Update 2016-13 [Member] | Retained Earnings [Member]            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Cumulative effect of new accounting principle in period of adoption     $ 0 $ (1,435) $ 0 $ 0
Reinsurance Recoverable [Domain] | Accounting Standards Update 2016-13 [Member] | Retained Earnings [Member]            
Liability for Claims and Claims Adjustment Expense [Line Items]            
Cumulative effect of new accounting principle in period of adoption     $ 0 $ 2,903 [2]    
[1] Includes an adjustment of $2.9 million related to our adoption of ASU 2016-13. Refer to Note 2. "Adoption of Accounting Pronouncements" for additional information.
[2] See Note 2. "Adoption of Accounting Pronouncements" for additional information regarding our adoption of ASU 2016-13.