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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, 2020
Mar. 31, 2019
Dec. 31, 2019
Dec. 31, 2018
Liability for Claims and Claims Adjustment Expense [Line Items]        
Cumulative effect of new accounting principle in period of adoption     $ 1,435  
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 407,276 397,253    
Total incurred loss and loss expense 400,324 386,579    
Paid loss and loss expense for claims occurring in the current year 70,610 73,964    
Paid loss and loss expense for claims occurring in the prior years 281,736 246,168    
Total paid loss and loss expense 352,346 320,132    
Net reserve for loss and loss expense, at end of period 3,568,075 3,422,927    
Add: reinsurance recoverable on unpaid loss and loss expense, at end of period 535,560 543,582    
Gross reserve for loss and loss expense, at end of period 4,103,635 3,966,509    
Loss Reserve for COVID-19 10,000      
Change in Historical Claim Rate Experience [Member]        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Prior year development (6,952) (10,674)    
Workers' Compensation Insurance [Member] | Change in Historical Claim Rate Experience [Member]        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Prior year development (10,000)      
Property Insurance [Member] | Change in Historical Claim Rate Experience [Member]        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Prior year development $ 3,000      
Workers Compensation [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   (8,000)    
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   $ (2,000)    
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     1,435 $ 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 [2]     $ (2,903)  
[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.