STOCK COMPENSATION PLANS (Schedule of Compensation Costs) (Details) - USD ($) $ / shares in Units, $ in Millions |
3 Months Ended | 12 Months Ended | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
Dec. 31, 2019 |
Sep. 30, 2019 |
Jun. 30, 2019 |
Mar. 31, 2019 |
Dec. 31, 2018 |
Sep. 30, 2018 |
Jun. 30, 2018 |
Mar. 31, 2018 |
Dec. 31, 2019 |
Dec. 31, 2018 |
Dec. 31, 2017 |
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| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Share-based Payment Arrangement, Expense, Tax Benefit | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | $ (3.7) | $ 0.0 | $ 0.0 |
| Share-based Payment Arrangement, Expense, after Tax | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | $ 13.9 | $ 15.1 | $ 18.2 |
| Earnings (loss) per common share attributable to Cliffs common shareholders - basic: | $ 0.23 | $ 0.34 | $ 0.59 | $ (0.08) | $ 2.06 | $ 1.47 | $ 0.55 | $ (0.29) | $ 1.06 | $ 3.80 | $ 1.28 |
| Earnings (loss) per common share attributable to Cliffs common shareholders - diluted: | $ 0.23 | $ 0.33 | $ 0.57 | $ (0.08) | $ 1.98 | $ 1.41 | $ 0.55 | $ (0.29) | 1.03 | 3.71 | 1.26 |
| Share-based Payment Arrangement [Member] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Earnings (loss) per common share attributable to Cliffs common shareholders - basic: | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | 0.05 | 0.05 | 0.06 |
| Earnings (loss) per common share attributable to Cliffs common shareholders - diluted: | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | $ 0.05 | $ 0.05 | $ 0.06 |
| Cost of goods sold and operating expenses [Member] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Costs and Expenses | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | $ 2.0 | $ 1.7 | $ 1.9 |
| Selling, General and Administrative Expenses [Member] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Costs and Expenses | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | 15.6 | 13.4 | 16.3 |
| Operating Income (Loss) [Member] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Costs and Expenses | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | $ 17.6 | $ 15.1 | $ 18.2 |
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- Definition Amount of expense for award under share-based payment arrangement. Excludes amount capitalized. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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- Definition Amount, after tax, of expense for award under share-based payment arrangement. No definition available.
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- Definition The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period. Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period. Reference 1: http://www.xbrl.org/2003/role/exampleRef
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- Definition Amount of tax benefit for recognition of expense of award under share-based payment arrangement. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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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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