Quarterly report pursuant to Section 13 or 15(d)

Commitments And Contingencies (Details)

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Commitments And Contingencies (Details) (USD $)
In Millions, unless otherwise specified
 9 Months Ended 12 Months Ended 9 Months Ended 12 Months Ended 9 Months Ended 12 Months Ended 9 Months Ended 12 Months Ended 3 Months Ended 12 Months Ended
Sep. 30, 2011
MT
Sep. 30, 2011
Bloom Lake [Member]
Dec. 31, 2012
Bloom Lake [Member]
Dec. 31, 2011
Bloom Lake [Member]
Sep. 30, 2011
Lower War Eagle [Member]
Dec. 31, 2011
Lower War Eagle [Member]
Sep. 30, 2011
Koolyanobbing [Member]
Dec. 31, 2012
Koolyanobbing [Member]
Dec. 31, 2011
Koolyanobbing [Member]
Dec. 31, 2010
Koolyanobbing [Member]
MT
Sep. 30, 2011
Empire [Member]
T
Sep. 30, 2011
Empire and Tilden [Member]
Dec. 31, 2012
Empire and Tilden [Member]
Dec. 31, 2011
Empire and Tilden [Member]
Sep. 30, 2011
Tilden [Member]
T
Sep. 30, 2011
Oak Grove [Member]
Dec. 31, 2011
Oak Grove II [Member]
Capital investment committed   $ 206.0         $ 246.0         $ 178.0       $ 31.0  
Gross post-project production capacity, tons 8,000,000                 11,000,000 3,000,000       2,000,000    
Capital investment paid   65.0     31.0   133.0         123.0       26.0  
Capital investment, future payments   615.0 68.0 73.0 49.0 10.0 272.0 46.0 67.0     264.0 24.0 31.0   55.0 5.0
Capital Investment, 2012 scheduled payments         $ 8