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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, 2018
Jun. 30, 2017
Liability for Catastrophe Claims [Line Items]    
Increase in current year loss and loss expense incurred $ 72,000  
Increase in non catastrophe property losses 44,000  
Non catastrophe property losses 188,700  
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 756,855 684,877
Total incurred loss and loss expense 751,269 659,031
Paid loss and loss expense for claims occurring in the current year 214,169 171,724
Paid loss and loss expense for claims occurring in the prior years 457,441 425,521
Total paid loss and loss expense 671,610 597,245
Net reserve for loss and loss expense, at end of period 3,265,044 3,142,305
Add: reinsurance recoverable on unpaid loss and loss expense, at end of period 539,321 588,916
Gross reserve for loss and loss expense, at end of period 3,804,365 3,731,221
Change in Historical Claim Rate Experience [Member]    
Liability for Catastrophe Claims [Line Items]    
Prior year development (5,586) (25,846)
Change in Historical Claim Rate Experience [Member] | Property Insurance Product Line [Member]    
Liability for Catastrophe Claims [Line Items]    
Prior year development 6,400  
Change in Historical Claim Rate Experience [Member] | Casualty Insurance Product Line [Member]    
Liability for Catastrophe Claims [Line Items]    
Prior year development (12,000)  
Change in Historical Claim Rate Experience [Member] | Casualty Insurance Product Line [Member] | Workers Compensation [Member]    
Liability for Catastrophe Claims [Line Items]    
Prior year development (33,000) (15,300)
Change in Historical Claim Rate Experience [Member] | Casualty Insurance Product Line [Member] | Commercial Automobile [Member]    
Liability for Catastrophe Claims [Line Items]    
Prior year development 15,000 21,000
Change in Historical Claim Rate Experience [Member] | Casualty Insurance Product Line [Member] | Excess And Surplus Operations [Member]    
Liability for Catastrophe Claims [Line Items]    
Prior year development $ 6,000  
Change in Historical Claim Rate Experience [Member] | Casualty Insurance Product Line [Member] | General Liability [Member]    
Liability for Catastrophe Claims [Line Items]    
Prior year development   (37,400)
Change in Historical Claim Rate Experience [Member] | Casualty Insurance Product Line [Member] | Personal Automobile [Member]    
Liability for Catastrophe Claims [Line Items]    
Prior year development   $ 4,000