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Reserve for Loss and Loss Expense (Rollforward of Reserve for Loss and Loss Expense) (Details) - USD ($)
$ in Thousands
 9 Months Ended
Sep. 30, 2022
Sep. 30, 2021
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 1,610,940 1,408,240
Total incurred loss and loss expense 1,566,930 1,340,293
Paid loss and loss expense for claims occurring in the current year 513,118 430,288
Paid loss and loss expense for claims occurring in the prior years 779,438 675,782
Total paid loss and loss expense 1,292,556 1,106,070
Net reserve for loss and loss expense, at end of period 4,276,636 3,940,309
Add: reinsurance recoverable on unpaid loss and loss expense, at end of period 688,641 670,031
Gross reserve for loss and loss expense, at end of period 4,965,277 4,610,340
Change in Historical Claim Rate Experience [Member]    
Liability for Claims and Claims Adjustment Expense [Line Items]    
Prior year development (44,010) (67,947)
Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member]    
Liability for Claims and Claims Adjustment Expense [Line Items]    
Prior year development (48,000) (66,000)
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 (40,000) (28,000)
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) (29,000)
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 (10,000)  
Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member] | Business Owners Policies [Member]    
Liability for Claims and Claims Adjustment Expense [Line Items]    
Prior year development (8,000) $ (2,000)
Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member] | Commercial Automobile [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 $ 4,000