Mine Closure and Asset Impairment Costs (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
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					(1,770)
Mine closure and asset impairment costs	$ 0	$ (2,194)	$ 525,762	$ 0	$ 523,568	$ 7,316