Mine Closure and Asset Impairment Costs (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Restructuring and Related Activities [Abstract]
Parts and supplies inventory writedown			$ 2,598
Impairment of property, plant and equipment			95,641
Impairment of coal properties and deferred development costs			403,279
Royalty obligations			11,546
Employee termination benefits			12,274
Curtailments	(2,200)		(1,770)
Mine closure and asset impairment costs	$ (2,194)		$ 523,568	$ 7,316