CAPITAL STOCK - Narrative (Details) $ / shares in Units, $ in Millions |
3 Months Ended | 6 Months Ended | ||||||||||
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Mar. 11, 2021
USD ($)
|
Dec. 09, 2020
shares
|
Apr. 24, 2020
USD ($)
|
Mar. 13, 2020
USD ($)
shares
|
Jun. 30, 2021
USD ($)
$ / shares
shares
|
Jun. 30, 2020
USD ($)
|
Jun. 30, 2021
USD ($)
$ / shares
shares
|
Jun. 30, 2020
USD ($)
|
Apr. 29, 2021
shares
|
Mar. 31, 2021
shares
|
Feb. 11, 2021
$ / shares
shares
|
Dec. 31, 2020
shares
|
|
Class of Stock [Line Items] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Common shares, issued (in shares) | Ìý | Ìý | Ìý | Ìý | 506,832,537 | Ìý | 506,832,537 | Ìý | Ìý | Ìý | 20,000,000 | 506,832,537 |
Debt Extinguished | $ | Ìý | Ìý | $ 736 | Ìý | $ 421 | $ 748 | $ 1,278 | $ 1,488 | Ìý | Ìý | Ìý | Ìý |
Common shares, par value (in dollars per share) | $ / shares | Ìý | Ìý | Ìý | Ìý | $ 0.125 | Ìý | $ 0.125 | Ìý | Ìý | Ìý | Ìý | Ìý |
Series B Participating Redeemable Preferred Stock, issued (in shares) | Ìý | Ìý | Ìý | Ìý | 583,273 | Ìý | 583,273 | Ìý | Ìý | Ìý | Ìý | 583,273 |
Series B Participating Redeemable Preferred Stock, outstanding (in shares) | Ìý | Ìý | Ìý | Ìý | 583,273 | Ìý | 583,273 | Ìý | Ìý | Ìý | Ìý | 583,273 |
Common shares, authorized (in shares) | Ìý | Ìý | Ìý | Ìý | 1,200,000,000 | Ìý | 1,200,000,000 | Ìý | 1,200,000,000 | 600,000,000 | Ìý | 600,000,000 |
9.875% 2025 Senior Secured Notes | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Class of Stock [Line Items] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Debt Extinguished | $ | $ 322 | Ìý | Ìý | Ìý | $ 25 | Ìý | $ 347 | Ìý | Ìý | Ìý | Ìý | Ìý |
Arcelor Mittal | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Class of Stock [Line Items] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Common shares sold (in shares) | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | 40,000,000 | Ìý |
ÐÇ¿Õ´«Ã½ Inc. | 9.875% 2025 Senior Secured Notes | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Class of Stock [Line Items] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Stated interest rate | Ìý | Ìý | Ìý | Ìý | 9.875% | Ìý | 9.875% | Ìý | Ìý | Ìý | Ìý | 9.875% |
AK Steel | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Class of Stock [Line Items] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Exchange ratio | Ìý | Ìý | Ìý | 0.400 | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Fair value of Cliffs common shares issued for AK Steel outstanding common stock | $ | Ìý | Ìý | Ìý | $ 618 | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Common Stock | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Class of Stock [Line Items] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Share price (in dollars per share) | $ / shares | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | $ 16.12 | Ìý |
Common Stock | ArcelorMittal USA | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Class of Stock [Line Items] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Stock issued during period, shares, acquisitions (in shares) | Ìý | 78,186,671 | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Common Stock | AK Steel | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Class of Stock [Line Items] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Stock issued during period, shares, acquisitions (in shares) | Ìý | Ìý | Ìý | 127,000,000 | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Exchange ratio | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | 0.400 | Ìý | Ìý | Ìý | Ìý | Ìý |
Series B Preferred Stock | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Class of Stock [Line Items] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Preferred stock authorized (in shares) | Ìý | Ìý | Ìý | Ìý | 4,000,000 | Ìý | 4,000,000 | Ìý | Ìý | Ìý | Ìý | Ìý |
Series B Participating Redeemable Preferred Stock, issued (in shares) | Ìý | Ìý | Ìý | Ìý | 583,273 | Ìý | 583,273 | Ìý | Ìý | Ìý | Ìý | Ìý |
Series B Participating Redeemable Preferred Stock, outstanding (in shares) | Ìý | Ìý | Ìý | Ìý | 583,273 | Ìý | 583,273 | Ìý | Ìý | Ìý | Ìý | Ìý |
Series B Preferred Stock | ArcelorMittal USA | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Class of Stock [Line Items] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Stock issued during period, shares, acquisitions (in shares) | Ìý | 583,273 | Ìý | Ìý | Ìý | Ìý | 583,273 | Ìý | Ìý | Ìý | Ìý | Ìý |
Series A Preferred Stock | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Class of Stock [Line Items] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Preferred stock authorized (in shares) | Ìý | Ìý | Ìý | Ìý | 3,000,000 | Ìý | 3,000,000 | Ìý | Ìý | Ìý | Ìý | Ìý |
Preferred stock outstanding (in shares) | Ìý | Ìý | Ìý | Ìý | 0 | Ìý | 0 | Ìý | Ìý | Ìý | Ìý | Ìý |
Preferred stock issued (in shares) | Ìý | Ìý | Ìý | Ìý | 0 | Ìý | 0 | Ìý | Ìý | Ìý | Ìý | Ìý |