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REVENUES - By Market and Product Line (Details) T in Thousands, $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 USD ($) T | Dec. 31, 2019 USD ($) T | Dec. 31, 2018 USD ($) T
Disaggregation of Revenue [Line Items] |  |  |
Revenues | $ 5,354 | $ 1,990 | $ 2,332
Realization of Deferred Revenue | 35 | 0 | 0
Customer Supply Agreements [Member] | Sales [Member] |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net | 122 | 78 | 426
Steelmaking [Member] |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenues | 4,965 | 1,990 | 2,332
Steelmaking [Member] | Hot-rolled steel |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenues | $ 386 | $ 0 | $ 0
Sales Volume (Net Tons) | T | 633 | 0 | 0
Steelmaking [Member] | Cold-rolled steel |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenues | $ 490 | $ 0 | $ 0
Sales Volume (Net Tons) | T | 682 | 0 | 0
Steelmaking [Member] | Coated steel |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenues | $ 1,747 | $ 0 | $ 0
Sales Volume (Net Tons) | T | 1,911 | 0 | 0
Steelmaking [Member] | Stainless and electrical steel |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenues | $ 868 | $ 0 | $ 0
Sales Volume (Net Tons) | T | 416 | 0 | 0
Steelmaking [Member] | Other steel products |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenues | $ 92 | $ 0 | $ 0
Sales Volume (Net Tons) | T | 141 | 0 | 0
Steelmaking [Member] | Iron products |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenues | $ 1,335 | $ 1,990 | $ 2,332
Sales Volume (Long Tons) | T | 11,707 | 18,583 | 20,563
Steelmaking [Member] | Other [Member] |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenues | $ 47 | $ 0 | $ 0
Other businesses |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenues | 389 | 0 | 0
Other businesses | Other [Member] |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenues | 389 | 0 | 0
Automotive [Member] | Steelmaking [Member] |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenues | 2,062 | 0 | 0
Automotive [Member] | Other businesses |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenues | 329 | 0 | 0
Infrastructure and manufacturing | Steelmaking [Member] |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenues | 784 | 0 | 0
Infrastructure and manufacturing | Other businesses |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenues | 34 | 0 | 0
Distributors and converters | Steelmaking [Member] |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenues | 696 | 0 | 0
Distributors and converters | Other businesses |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenues | 26 | 0 | 0
Steel producers | Steelmaking [Member] |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Revenues | $ 1,423 | $ 1,990 | $ 2,332

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