| ACQUISITIONS - AK Steel Intangible Assets and Liabilities (Details) - AK Steel $ in Millions | 6 Months Ended | 
|---|---|
| Jun. 30, 2021  USD ($) | |
| Acquired Finite-Lived Intangible Assets [Line Items] | |
| Total identifiable intangible assets | $ 148 | 
| Weighted Average Life (In Years) | 17 years | 
| Above-market supply contracts | $ (71) | 
| Above-market supply contract, Weighted Average Useful Life (in years) | 12 years | 
| Customer relationships | |
| Acquired Finite-Lived Intangible Assets [Line Items] | |
| Total identifiable intangible assets | $ 77 | 
| Weighted Average Life (In Years) | 18 years | 
| Developed technology | |
| Acquired Finite-Lived Intangible Assets [Line Items] | |
| Total identifiable intangible assets | $ 60 | 
| Weighted Average Life (In Years) | 17 years | 
| Trade names and trademarks | |
| Acquired Finite-Lived Intangible Assets [Line Items] | |
| Total identifiable intangible assets | $ 11 | 
| Weighted Average Life (In Years) | 10 years | 
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- Definition Weighted average amortization period of finite-lived intangible liabilities assumed either individually or as part of a group of liabilities, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. No definition available. 
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- Definition Amount of increase in liabilities, excluding financial liabilities, lacking physical substance with a definite life, from an acquisition. No definition available. 
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. No definition available. 
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- Definition Weighted average amortization period of finite-lived intangible assets acquired either individually or as part of a group of assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef 
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- Definition Amount of increase in assets, excluding financial assets, lacking physical substance with a definite life, from an acquisition. Reference 1: http://www.xbrl.org/2003/role/disclosureRef 
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