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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
 6 Months Ended
Jun. 30, 2017
Jun. 30, 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 684,877 624,159
Incurred losses and loss expenses for claims occurring in the prior year (25,846) (28,536)
Total incurred losses and loss expenses 659,031 595,623
Paid losses and loss expenses for claims occurring in the current year 171,724 146,849
Paid losses and loss expenses for claims occurring in the prior years 425,521 402,255
Total paid losses and loss expenses 597,245 549,104
Net reserves for losses and loss expenses, at end of period 3,142,305 3,013,228
Add: reinsurance recoverable on unpaid losses and loss expenses, at end of period 588,916 576,941
Gross reserves for losses and loss expenses, at end of period 3,731,221 3,590,169
Change in Historical Claim Rate Experience [Member]    
Liability for Catastrophe Claims [Line Items]    
Incurred losses and loss expenses for claims occurring in the prior year 25,800 28,500
Change in Historical Claim Rate Experience [Member] | General Liability [Member]    
Liability for Catastrophe Claims [Line Items]    
Incurred losses and loss expenses for claims occurring in the prior year 37,400 22,000
Change in Historical Claim Rate Experience [Member] | Workers Compensation [Member]    
Liability for Catastrophe Claims [Line Items]    
Incurred losses and loss expenses for claims occurring in the prior year 15,300 21,000
Change in Historical Claim Rate Experience [Member] | Commercial Automobile [Member]    
Liability for Catastrophe Claims [Line Items]    
Incurred losses and loss expenses for claims occurring in the prior year 21,000 13,000
Change in Historical Claim Rate Experience [Member] | Personal Automobile [Member]    
Liability for Catastrophe Claims [Line Items]    
Incurred losses and loss expenses for claims occurring in the prior year $ 4,000  
Change in Historical Claim Rate Experience [Member] | Excess And Surplus Operations [Member]    
Liability for Catastrophe Claims [Line Items]    
Incurred losses and loss expenses for claims occurring in the prior year   $ 3,000