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PENSION AND OTHER POSTRETIREMENT BENEFITS - Obligations and Funded Status (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract] |  |  |
Non-current liabilities | $ (4,113.0) | $ (312.0) |
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract] |  |  |
Defined Benefit Plan, Accumulated Benefit Obligation | 6,537.0 | 1,010.0 |
ArcelorMittal USA [Member] |  |  |
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] |  |  |
Acquired through business combinations | 3,294.0 |  |
AK Steel [Member] |  |  |
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] |  |  |
Acquired through business combinations | 949.0 |  |
Pension Plan [Member] |  |  |
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] |  |  |
Benefit obligations - beginning of year | 1,021.0 | 906.0 |
Service cost (excluding expense) | 23.0 | 17.0 | $ 19.0
Interest cost | 64.0 | 35.0 | 30.0
Actuarial (gain) loss | 162.0 | 112.0 |
Benefits paid | (146.0) | (62.0) |
Participant contributions | 0.0 | 0.0 |
Acquired through business combinations | 5,535.0 | 0.0 |
Effect of settlement | (94.0) | 0.0 |
Other | 0.0 | 13.0 |
Benefit obligations - end of year | 6,565.0 | 1,021.0 | 906.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |  |
Fair value of plan assets — beginning of year | 749.0 | 687.0 |
Actual return on plan assets | 472.0 | 98.0 |
Participant contributions | 0.0 | 0.0 |
Employer contributions | 50.0 | 16.0 |
Benefits paid | (146.0) | (62.0) |
Acquired through business combinations | 4,301.0 | 0.0 |
Effect of settlement | (94.0) | 0.0 |
Other | 0.0 | 10.0 |
Fair value of plan assets — end of year | 5,332.0 | 749.0 | 687.0
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract] |  |  |
Assets for Plan Benefits, Defined Benefit Plan | 3.0 | 0.0 |
Current liabilities | (12.0) | 0.0 |
Non-current liabilities | (1,224.0) | (272.0) |
Total amount recognized | (1,233.0) | (272.0) |
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract] |  |  |
Net actuarial loss | 164.0 | 382.0 |
Prior service cost (credit) | 6.0 | 7.0 |
Net amount recognized | 170.0 | 389.0 |
Funded (Unfunded) Status | (1,233.0) | (272.0) |
Other Postretirement Benefits Plan [Member] |  |  |
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] |  |  |
Benefit obligations - beginning of year | 255.0 | 242.0 |
Service cost (excluding expense) | 8.0 | 2.0 | 2.0
Interest cost | 19.0 | 10.0 | 8.0
Actuarial (gain) loss | 14.0 | 19.0 |
Benefits paid | (89.0) | (26.0) |
Participant contributions | 22.0 | 6.0 |
Acquired through business combinations | 3,528.0 | 0.0 |
Effect of settlement | 0.0 | 0.0 |
Other | 0.0 | 2.0 |
Benefit obligations - end of year | 3,757.0 | 255.0 | 242.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |  |
Fair value of plan assets — beginning of year | 260.0 | 240.0 |
Actual return on plan assets | 45.0 | 35.0 |
Participant contributions | 17.0 | 1.0 |
Employer contributions | 30.0 | 3.0 |
Benefits paid | (88.0) | (19.0) |
Acquired through business combinations | 519.0 | 0.0 |
Effect of settlement | 0.0 | 0.0 |
Other | 0.0 | 0.0 |
Fair value of plan assets — end of year | 783.0 | 260.0 | $ 240.0
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract] |  |  |
Assets for Plan Benefits, Defined Benefit Plan | 54.0 | 49.0 |
Current liabilities | (139.0) | (4.0) |
Non-current liabilities | (2,889.0) | (40.0) |
Total amount recognized | (2,974.0) | 5.0 |
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract] |  |  |
Net actuarial loss | 56.0 | 73.0 |
Prior service cost (credit) | (6.0) | (8.0) |
Net amount recognized | 50.0 | 65.0 |
Funded (Unfunded) Status | $ (2,974.0) | $ 5.0 |

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