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Transactions with Related Parties - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Related Party Transaction [Line Items]          
Net sales to related parties $ 367 $ 378 $ 1,112 $ 1,072  
Accounts payable to related parties 111   111   $ 81
Outside processing services          
Related Party Transaction [Line Items]          
Purchases from related parties 7 8 23 23  
Taconite pellets          
Related Party Transaction [Line Items]          
Purchases from related parties 27 $ 23 78 $ 66  
PRO-TEC Coating Company          
Related Party Transaction [Line Items]          
Accounts payable to related parties 109   109   80
Other equity investees          
Related Party Transaction [Line Items]          
Accounts payable to related parties $ 2   $ 2   $ 1