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	Mar. 31, 2015	Dec. 31, 2015	Oct. 01, 2016	Dec. 31, 2015	Dec. 31, 2014
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment and mine closure costs					$ 488,900	$ 2,100,000	$ 19,100		$ 2,600,000
Predecessor [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Coal lands and mineral rights										$ 74,144	$ 2,210,488	$ 0
Plant and equipment										0	199,107	1,512
Deferred development										0	159,474	0
Prepaid royalties										3,406	41,990	15,356
Equity investments										40,920	21,325	0
Inventories										0	66	0
Other										10,797	(4,147)	7,245
Asset impairment and mine closure costs		$ 46	$ 43,701	$ 85,520	$ 488,865	$ 2,120,292	$ 19,146	$ 0		$ 129,267	$ 2,628,303	$ 24,113
Successor [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Coal lands and mineral rights	$ 0
Plant and equipment	0
Deferred development	0
Prepaid royalties	0
Equity investments	0
Inventories	0
Other	0
Asset impairment and mine closure costs	$ 0