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Reserve for Loss and Loss Expense Reserve for Loss and Loss Expense (Rollforward of Reserve for Loss and Loss Expense) (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Liability for Catastrophe Claims [Line Items]    
Increase in loss and loss expense incurred $ 57,500  
Increase in catastrophe losses 13,800  
Catastrophe losses 26,040  
Increase in non catastrophe property losses 34,300  
Non catastrophe property losses 105,700  
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 388,801 331,331
Total incurred loss and loss expense 384,941 317,472
Paid loss and loss expense for claims occurring in the current year 79,345 54,708
Paid loss and loss expense for claims occurring in the prior years 248,738 235,742
Total paid loss and loss expense 328,083 290,450
Net reserve for loss and loss expense, at end of period 3,242,243 3,107,541
Add: reinsurance recoverable on unpaid loss and loss expense, at end of period 549,837 571,930
Gross reserve for loss and loss expense, at end of period 3,792,080 3,679,471
Change in Historical Claim Rate Experience [Member]    
Liability for Catastrophe Claims [Line Items]    
Prior Year Claims and Claims Adjustment Expense (3,860) (13,859)
Change in Historical Claim Rate Experience [Member] | Property Insurance Product Line [Member]    
Liability for Catastrophe Claims [Line Items]    
Prior Year Claims and Claims Adjustment Expense 4,100  
Change in Historical Claim Rate Experience [Member] | Casualty Insurance Product Line [Member]    
Liability for Catastrophe Claims [Line Items]    
Prior Year Claims and Claims Adjustment Expense (8,000)  
Change in Historical Claim Rate Experience [Member] | Casualty Insurance Product Line [Member] | Workers Compensation [Member]    
Liability for Catastrophe Claims [Line Items]    
Prior Year Claims and Claims Adjustment Expense (16,000)  
Change in Historical Claim Rate Experience [Member] | Casualty Insurance Product Line [Member] | Commercial Automobile [Member]    
Liability for Catastrophe Claims [Line Items]    
Prior Year Claims and Claims Adjustment Expense $ 8,000 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 Claims and Claims Adjustment Expense   $ (22,400)