Asset Impairments and Mine Closure Costs (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Restructuring Cost and Reserve [Line Items]
Asset impairment and mine closure costs	$ 43,701	$ 85,500	$ 19,146	$ 129,221	$ 19,146
Impairment charge on investment		2,900
Curtailments	3,600
Severance expense	2,100	5,100
Longview, Washington [Member]
Restructuring Cost and Reserve [Line Items]
Impairment charge on investment	$ 38,000
Kentucky [Member]
Restructuring Cost and Reserve [Line Items]
Impaired coal reserves and surface land		74,100
Cumberland River [Member]
Restructuring Cost and Reserve [Line Items]
Impairment charge			5,600
Royalty Agreements [Member]
Restructuring Cost and Reserve [Line Items]
Impairment charge		$ 3,400
Royalty Agreements [Member] | Mountain Laurel, Spruce and Briar Branch [Member]
Restructuring Cost and Reserve [Line Items]
Impairment charge			$ 12,200