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Risk Concentrations (Narrative) (Details)
$ in Thousands, T in Millions
3 Months Ended 9 Months Ended 12 Months Ended
Dec. 31, 2018
USD ($)
Sep. 30, 2018
USD ($)
Jun. 30, 2018
USD ($)
Mar. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Sep. 30, 2017
USD ($)
Jun. 30, 2017
USD ($)
Mar. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Oct. 01, 2016
USD ($)
Dec. 31, 2018
USD ($)
T
Dec. 31, 2017
USD ($)
Concentration Risk [Line Items]                        
Trade accounts receivable $ 200,904       $ 172,604           $ 200,904 $ 172,604
Tons of coal sold | T                     96.6  
Percentage of tons sold under long term contract                     60.00%  
Long term contracts percentage of company revenue                     47.00%  
Long-term contracts, life term, minimum                     1 year  
Long-term contracts, life term, maximum                     4 years  
Revenues 650,963 $ 633,180 $ 592,349 $ 575,295 560,244 $ 613,538 $ 549,866 $ 600,975 $ 575,688   $ 2,451,787 2,324,623
Europe                        
Concentration Risk [Line Items]                        
Revenues                 61,169   559,165 388,926
Asia                        
Concentration Risk [Line Items]                        
Revenues                 55,634   452,711 264,503
Central and South America                        
Concentration Risk [Line Items]                        
Revenues                 13,224   79,085 30,982
Africa                        
Concentration Risk [Line Items]                        
Revenues                 0   17,567 14,901
Brokered Sales                        
Concentration Risk [Line Items]                        
Revenues                 $ 0   2,372 3,150
Predecessor                        
Concentration Risk [Line Items]                        
Revenues                   $ 1,398,709    
Predecessor | Europe                        
Concentration Risk [Line Items]                        
Revenues                   112,986    
Predecessor | Asia                        
Concentration Risk [Line Items]                        
Revenues                   68,536    
Predecessor | Central and South America                        
Concentration Risk [Line Items]                        
Revenues                   41,861    
Predecessor | Africa                        
Concentration Risk [Line Items]                        
Revenues                   0    
Predecessor | Brokered Sales                        
Concentration Risk [Line Items]                        
Revenues                   $ 0    
Electric utilities                        
Concentration Risk [Line Items]                        
Receivables 73,900       72,900           $ 73,900 $ 72,900
Percentage of total trade accounts receivable                     37.00% 42.00%
Domestic and foreign steel producers                        
Concentration Risk [Line Items]                        
Trade accounts receivable $ 126,500       $ 99,400           $ 126,500 $ 99,400
Trade receivables, percentage 63.00%       58.00%           63.00% 58.00%