Asset Impairment and Restructuring (Details) (USD $)	3 Months Ended	12 Months Ended
	Dec. 31, 2012 item	Dec. 31, 2012 item
Asset Impairment and Restructuring
Number of Canadian mines at which production was reduced	2	2
Number of Canadian mines	3	3
Restructuring and impairment charge	$ 9,100,000	$ 1,113,479,000
Severance and other obligations	6,000,000
Impairment of property, plant and equipment	3,100,000
Write-off capitalized exploratory costs associated with natural gas exploration project, net of taxes		40,000,000
Write-off capitalized exploratory costs associated with natural gas exploration project, net of taxes		$ 25,000,000