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Long Term Retention Plan (Long Term Retention Plans Accrued Compensation Expense) (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Long term retention plan $ 10,914 $ 6,666 $ 27,706 $ 28,734
LTRP 2009 [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Long term retention plan       29
LTRP 2010 [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Long term retention plan   147 24 891
LTRP 2011 [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Long term retention plan 318 229 806 1,422
LTRP 2012 [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Long term retention plan 526 315 1,325 1,945
LTRP 2013 [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Long term retention plan 846 711 2,268 3,809
LTRP 2014 [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Long term retention plan 1,056 772 2,768 3,782
LTRP 2015 [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Long term retention plan 1,474 1,063 3,790 4,680
LTRP 2016 [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Long term retention plan 2,131 1,606 5,547 6,717
LTRP 2017 [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Long term retention plan 2,074 $ 1,823 5,587 $ 5,459
LTRP 2018 [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Long term retention plan $ 2,489   $ 5,591