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Reserve for Losses and Loss Expenses Reserves for Losses and Loss Expenses (Rollforward of Reserves for Losses and Loss Expenses) (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Liability for Catastrophe Claims [Line Items]    
Gross reserves for losses and loss expenses, at beginning of year $ 3,691,719 $ 3,517,728
Less: reinsurance recoverable on unpaid losses and loss expenses, at beginning of year 611,200 551,019
Net reserves for losses and loss expenses, at beginning of year 3,080,519 2,966,709
Incurred losses and loss expenses for claims occurring in the current year 331,331 312,778
Prior Year Claims and Claims Adjustment Expense (13,859) (15,634)
Total incurred losses and loss expenses 317,472 297,144
Paid losses and loss expenses for claims occurring in the current year 54,708 44,310
Paid losses and loss expenses for claims occurring in the prior years 235,742 212,368
Total paid losses and loss expenses 290,450 256,678
Net reserves for losses and loss expenses, at end of period 3,107,541 3,007,175
Add: reinsurance recoverable on unpaid losses and loss expenses, at end of period 571,930 568,321
Gross reserves for losses and loss expenses, at end of period 3,679,471 3,575,496
Change in Historical Claim Rate Experience [Member]    
Liability for Catastrophe Claims [Line Items]    
Prior Year Claims and Claims Adjustment Expense 13,900 15,600
Change in Historical Claim Rate Experience [Member] | General Liability [Member]    
Liability for Catastrophe Claims [Line Items]    
Prior Year Claims and Claims Adjustment Expense 22,400 11,000
Change in Historical Claim Rate Experience [Member] | Commercial Automobile [Member]    
Liability for Catastrophe Claims [Line Items]    
Prior Year Claims and Claims Adjustment Expense 6,000 5,000
Change in Historical Claim Rate Experience [Member] | Personal Automobile [Member]    
Liability for Catastrophe Claims [Line Items]    
Prior Year Claims and Claims Adjustment Expense $ 2,000  
Change in Historical Claim Rate Experience [Member] | Workers Compensation [Member]    
Liability for Catastrophe Claims [Line Items]    
Prior Year Claims and Claims Adjustment Expense   12,000
Change in Historical Claim Rate Experience [Member] | Excess And Surplus Operations [Member]    
Liability for Catastrophe Claims [Line Items]    
Prior Year Claims and Claims Adjustment Expense   $ 1,000