Discontinued Operations (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) T Rate	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Impairment of Long-Lived Assets to be Disposed of	$ 0	$ 828,905	$ 0
Fola/Miller Creek [Member]
Cash Paid to Close Deal	28,271
Escrow Deposits Related to Property Sales	17,200
Gain (Loss) on Sale of Assets and Asset Impairment Charges	53,130
Impairment of Long-Lived Assets to be Disposed of	$ 355,681
Buchanan and Other Assets [Member]
Royalty Percent | Rate	20.00%
Royalty Per Ton - Year 1 | T	75.00
Royalty Per Ton - Year 2 | T	78.75
Royalty Per Ton - Year 3 | T	82.69
Royalty Per Ton - Year 4 | T	86.82
Royalty Per Ton - Year 5 | T	91.16
Proceeds from Sale of Property Held-for-sale	$ 402,799
Discontinued Operation, Gain (Loss) from Disposal of Discontinued Operation, before Income Tax	$ (38,364)