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Loss And Loss Adjustment Expense Reserves (Activity In The Reserves For Losses And Loss Adjustment Expenses) (Details) - USD ($)
$ in Thousands
 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]        
Gross reserves at January 1 $ 1,510,613 $ 1,290,248    
Less reinsurance recoverables on unpaid losses 50,027 10,798 $ 64,001 $ 13,161
Net reserves at January 1 1,446,612 1,277,087    
Incurred losses and loss adjustment expenses related to:        
Current year 1,781,452 1,772,203    
Prior years 70,399 18,347    
Total incurred losses and loss adjustment expenses 1,851,851 1,790,550    
Loss and loss adjustment expense payments related to:        
Current year 1,072,758 1,099,656    
Prior years 695,678 606,615    
Total payments 1,768,436 1,706,271    
Net reserves at September 30 1,530,027 1,361,366    
Gross reserves at September 30 $ 1,580,054 $ 1,372,164