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Related Party Transactions - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended 9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2013
Sep. 30, 2012
Dec. 31, 2012
Related Party Transaction [Line Items]          
Net sales to related parties $ 275 $ 342 $ 863 $ 1,003  
Accounts payable to related parties 93   93   78
Outside processing services
         
Related Party Transaction [Line Items]          
Purchases from related parties 18 16 53 44  
Taconite pellets
         
Related Party Transaction [Line Items]          
Purchases from related parties 63 84 180 204  
PRO-TEC Coating Company
         
Related Party Transaction [Line Items]          
Accounts payable to related parties 89   89   75
Other equity investees
         
Related Party Transaction [Line Items]          
Accounts payable to related parties $ 4   $ 4   $ 3