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Mine Closing, Reclamation, and Gas Well Closing (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Asset Retirement Obligation, Beginning of Period $ 429,967 $ 454,832
Asset Retirement Obligation, Cash Paid to Settle (15,882) (18,033)
Accretion Expense 23,924 32,438
Asset Retirement Obligation, End of Period 473,544 429,967
Asset Retirement Obligation, Other Change (785) (5,286)
Asset Retirement Obligation, Revision of Estimate $ 36,320 (33,984)
Jones Fork Mine [Member]    
Asset Retirement Obligation, Other Change   (2,355)
Hamilton No. 1 and 2 Mines [Member]    
Asset Retirement Obligation, Other Change   $ (2,133)