Mine Closure and Asset Impairment Costs (Narrative) (Details) (USD $)	3 Months Ended				12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2012 mining_complex	Dec. 31, 2011	Dec. 31, 2010
Restructuring and Related Activities [Abstract]
Restructuring, Settlement and Impairment Provisions	$ 0	$ (2,194,000)	$ 525,762,000	$ 0	$ 523,568,000	$ 7,316,000
number of mining complexes temporarily idled					1
number of mining complexes closed					4
Long-Lived Assets to be Abandoned, Carrying Value of Asset	51,000,000				51,000,000
Curtailments					$ (1,770,000)
Restructuring and Related Cost, Number of Positions Eliminated					750