Quarterly report pursuant to Section 13 or 15(d)

Basis Of Presentation And Significant Accounting Policies (Schedule Of Subsidiaries) (Detail)

v2.4.0.6
Basis Of Presentation And Significant Accounting Policies (Schedule Of Subsidiaries) (Detail)
9 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Northshore [Member]
   
Related Party Transaction [Line Items]    
Location Minnesota  
Ownership interest, percent 100.00%  
Operation Iron Ore  
United Taconite [Member]
   
Related Party Transaction [Line Items]    
Location Minnesota  
Ownership interest, percent 100.00%  
Operation Iron Ore  
Wabush [Member]
   
Related Party Transaction [Line Items]    
Location Labrador/Quebec, Canada  
Ownership interest, percent 100.00%  
Operation Iron Ore  
Bloom Lake [Member]
   
Related Party Transaction [Line Items]    
Location Quebec, Canada  
Ownership interest, percent 75.00% 75.00%
Operation Iron Ore  
Tilden [Member]
   
Related Party Transaction [Line Items]    
Location Michigan  
Ownership interest, percent 85.00% 85.00%
Operation Iron Ore  
Empire [Member]
   
Related Party Transaction [Line Items]    
Location Michigan  
Ownership interest, percent 79.00% 79.00%
Operation Iron Ore  
Koolyanobbing [Member]
   
Related Party Transaction [Line Items]    
Location Western Australia  
Ownership interest, percent 100.00%  
Operation Iron Ore  
Pinnacle [Member]
   
Related Party Transaction [Line Items]    
Location West Virginia  
Ownership interest, percent 100.00%  
Operation Coal  
Oak Grove [Member]
   
Related Party Transaction [Line Items]    
Location Alabama  
Ownership interest, percent 100.00%  
Operation Coal  
CLCC [Member]
   
Related Party Transaction [Line Items]    
Location West Virginia  
Ownership interest, percent 100.00%  
Operation Coal