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Acquisitions - Unaudited Pro Forma Information for U. S. Steel Including the Results of the Big River Steel Acquisition (Details) - Big River Steel - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Business Acquisition [Line Items]    
Net sales $ 3,736 $ 2,994
Net earnings (loss) $ 18 $ (367)