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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, 2018
Sep. 30, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]    
Increase in current year loss and loss expense incurred $ 111,000  
Increase in non catastrophe property losses 61,900  
Non catastrophe property losses 278,500  
Gross reserve for loss and loss expense, at beginning of year 3,771,240 $ 3,691,719
Less: reinsurance recoverable on unpaid loss and loss expense, at beginning of year 585,855 611,200
Net reserve for loss and loss expense, at beginning of year 3,185,385 3,080,519
Incurred loss and loss expense for claims occurring in the current year 1,148,032 1,037,079
Total incurred loss and loss expense 1,130,468 1,003,618
Paid loss and loss expense for claims occurring in the current year 369,036 314,686
Paid loss and loss expense for claims occurring in the prior years 610,734 581,186
Total paid loss and loss expense 979,770 895,872
Net reserve for loss and loss expense, at end of period 3,336,083 3,188,265
Add: reinsurance recoverable on unpaid loss and loss expense, at end of period 589,072 647,535
Gross reserve for loss and loss expense, at end of period 3,925,155 3,835,800
Change in Historical Claim Rate Experience [Member]    
Liability for Claims and Claims Adjustment Expense [Line Items]    
Prior year development (17,564) (33,461)
Property Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member]    
Liability for Claims and Claims Adjustment Expense [Line Items]    
Prior year development 6,400  
Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member]    
Liability for Claims and Claims Adjustment Expense [Line Items]    
Prior year development (24,000)  
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 (8,000) (48,300)
Excess And Surplus Operations [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 12,000 10,000
Personal 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   4,000
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 25,000 26,000
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 $ (53,000) $ (29,300)