Asset Impairments and Mine Closure Costs (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2016	Mar. 31, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Restructuring Cost and Reserve [Line Items]
Asset impairment and mine closure costs	$ 46	$ 129,300	$ 2,120,292	$ 129,267	$ 2,139,438
Impairment charge on investment	2,900
Severance expense	7,200
Curtailments	3,600
Kentucky [Member]
Restructuring Cost and Reserve [Line Items]
Impaired coal reserves and surface land	74,100			$ 74,100
Longview, Washington [Member]
Restructuring Cost and Reserve [Line Items]
Impairment charge on investment	38,000
Royalty Agreements [Member]
Restructuring Cost and Reserve [Line Items]
Impairment charge	$ 3,400