Impairment Charges and Mine Closure Costs (Summary Asset Impairments and Mine Closure Costs) (Details) - USD ($) $ in Thousands	3 Months Ended									6 Months Ended	12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Sep. 30, 2013	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Impairment Charges and Mine Closure Costs [Abstract]
Coal lands and mineral rights											$ 2,210,488	$ 0	$ 79,094
Plant and equipment									$ 142,800		199,107	1,512	36,296
Deferred development											159,474	0	13,451
Prepaid royalties											41,990	15,356	1,104
Equity investments											21,325	0	28,947
Notes receivable											0	0	49,203
Inventories											66	0	12,765
Other											(4,147)	7,245	19
Asset impairment and mine closure costs	$ 488,865	$ 2,120,292	$ 19,146	$ 0	$ 17,541	$ 5,060	$ 1,512	$ 0		$ 2,600,000	$ 2,628,303	$ 24,113	$ 220,879