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ACQUISITIONS - Preliminary purchase price allocation (Details) - USD ($) $ in Millions | 10 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Dec. 31, 2020 | Dec. 31, 2020 | Mar. 13, 2020 | Dec. 31, 2019
Business Acquisition [Line Items] |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Current Assets, Income Tax Receivable | $ 12 | $ 12 | $ 3 |
Goodwill | 1,406 | 1,406 |  | $ 2
ArcelorMittal USA [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents | 35 | 35 |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Receivables | 349 | 349 |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Inventory | 2,115 | 2,115 |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Other | 22 | 22 |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment | 4,017 | 4,017 |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets | 158 | 158 |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Accounts Payable | (758) | (758) |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Accrued Employment Costs | (271) | (271) |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, State and Local Taxes | (76) | (76) |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Pension and OPEB Liabilities | (109) | (109) |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other | (322) | (322) |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Pension and OPEB Liabilities | (3,195) | (3,195) |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other | (598) | (598) |  |
Business Combination, Acquisition of Less than 100 Percent, Noncontrolling Interest, Fair Value | (13) | (13) |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net | 1,366 | 1,366 |  |
Goodwill | 1,230 | 1,230 |  |
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Less Noncontrolling Interest | 2,596 | 2,596 |  |
AK Steel [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents | 39 | 39 | 38 |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Receivables | 664 | 664 | 666 |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Inventory | 1,320 | 1,320 | 1,563 |
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Current Assets, Income Tax Receivable | 3 | 3 |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Other | 49 | 49 | 65 |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment | 2,312 | 2,312 | 2,184 |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Income Taxes | 30 | 30 | 0 |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets | 472 | 472 | 475 |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Accounts Payable | (644) | (644) | (636) |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Accrued Employment Costs | (93) | (93) | (94) |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, State and Local Taxes | (31) | (31) | (35) |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Pension and OPEB Liabilities | (78) | (78) | (75) |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other | (196) | (196) | (201) |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt | 1,179 | 1,179 | 1,179 |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Pension and OPEB Liabilities | (871) | (871) | (873) |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other | (435) | (435) | (507) |
Business Combination, Acquisition of Less than 100 Percent, Noncontrolling Interest, Fair Value | (1) | (1) | 0 |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net | 1,361 | 1,361 | 1,394 |
Goodwill | 174 | 174 | 141 |
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Less Noncontrolling Interest | 1,535 | 1,535 | $ 1,535 |
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Cash and Cash Equivalents | 1 |  |  |
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Accounts Receivable | (2) |  |  |
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Inventory | (243) |  |  |
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Income Tax Receivables | 0 |  |  |
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Other Current Assets | (16) |  |  |
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Property, Plant, and Equipment | 128 |  |  |
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Deferred Income Taxes | 30 |  |  |
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Other Noncurrent Assets | (3) |  |  |
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Accounts Payable | (8) |  |  |
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Accrued Employment Cost | 1 |  |  |
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, State and Local Income Taxes | 4 |  |  |
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Retirement Benefit Liabilities | (3) |  |  |
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Other Current Liabilities | 5 |  |  |
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Long-term Debt | 0 |  |  |
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Retirement Benefit Liabilities, Noncurrent | 2 |  |  |
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Other Noncurrent Liabilities | 72 |  |  |
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Noncontrlling Interest | (1) |  |  |
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Net Identifiable Assets | (33) |  |  |
Goodwill, Purchase Accounting Adjustments | 33 |  |  |
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Consideration Transferred | $ 0 |  |  |
AK Steel [Member] | Inventories [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Inventory |  | 243 |  |
AK Steel [Member] | Cost of goods sold |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustments Related to Previous Period |  | $ 8 |  |

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