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Inventories (Details) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Inventory Disclosure [Abstract]    
Coal $ 48,753 $ 37,268
Repair parts and supplies 73,683 76,194
Inventories 122,436 113,462
Allowance for slow-moving and obsolete inventories $ (200) $ 0