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Impairment Charges and Mine Closure Costs (Summary Asset Impairments and Mine Closure Costs) (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended 9 Months Ended 12 Months Ended
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Dec. 31, 2015
Oct. 01, 2016
Dec. 31, 2017
Dec. 31, 2015
Impaired Long-Lived Assets Held and Used [Line Items]                      
Asset impairment and mine closure costs $ 0       $ 500,000 $ 2,100,000 $ 19,100 $ 2,600,000   $ 0  
Successor                      
Impaired Long-Lived Assets Held and Used [Line Items]                      
Coal lands and mineral rights 0                 0  
Plant and equipment 0                 0  
Deferred development 0                 0  
Prepaid royalties 0                 0  
Equity investments 0                 0  
Inventories 0                 0  
Other 0                 0  
Asset impairment and mine closure costs $ 0                 $ 0  
Predecessor                      
Impaired Long-Lived Assets Held and Used [Line Items]                      
Coal lands and mineral rights                 $ 74,144   $ 2,210,488
Plant and equipment                 0   199,107
Deferred development                 0   159,474
Prepaid royalties                 3,406   41,990
Equity investments                 40,920   21,325
Inventories                 0   66
Other                 10,797    
Other                     (4,147)
Asset impairment and mine closure costs   $ 46 $ 43,701 $ 85,520         $ 129,267   $ 2,628,303