Asset Impairments and Mine Closure Costs (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Restructuring Cost and Reserve [Line Items]
Asset impairment and mine closure costs	$ 85,520	$ 0
Other-than-temporary-impairment charge on an available-for-sale security	2,900
KENTUCKY
Restructuring Cost and Reserve [Line Items]
Impaired coal reserves and surface land	74,100
Severance expense	5,100
Royalty Agreements [Member]
Restructuring Cost and Reserve [Line Items]
Impairment charge	$ 3,400