QUARTERLY RESULTS OF OPERATIONS (Schedule of Quarterly Financial Information) (Details) - USD ($) $ / shares in Units, shares in Millions, $ 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 |
|
| Revenues from product sales and services | $ 534.1 | $ 555.6 | $ 743.2 | $ 157.0 | $ 696.3 | $ 741.8 | $ 714.3 | $ 180.0 | $ 1,989.9 | $ 2,332.4 | $ 1,866.0 |
| Sales margin | 126.9 | 154.9 | 263.0 | 30.9 | 202.0 | 261.6 | 284.5 | 61.5 | 575.7 | 809.6 | 467.6 |
| Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent | 63.4 | 91.8 | 161.4 | (22.1) | 624.1 | 199.8 | 229.4 | (13.4) | 294.5 | 1,039.9 | 364.5 |
| Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent | (0.2) | (0.9) | (0.6) | 0.0 | (14.6) | 238.0 | (64.3) | (70.9) | (1.7) | 88.2 | 2.5 |
| Net Income (Loss) Available to Common Stockholders, Basic | $ 63.2 | $ 90.9 | $ 160.8 | $ (22.1) | $ 609.5 | $ 437.8 | $ 165.1 | $ (84.3) | $ 292.8 | $ 1,128.1 | $ 367.0 |
| Earnings (loss) per common share attributable to Cliffs common shareholders - basic: | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Continuing operations | $ 0.23 | $ 0.34 | $ 0.59 | $ (0.08) | $ 2.11 | $ 0.67 | $ 0.77 | $ (0.05) | $ 1.07 | $ 3.50 | $ 1.27 |
| Discontinued operations | 0 | 0 | 0 | 0 | (0.05) | 0.80 | (0.22) | (0.24) | (0.01) | 0.30 | 0.01 |
| 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: | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Continuing operations | 0.23 | 0.33 | 0.57 | (0.08) | 2.03 | 0.64 | 0.76 | (0.05) | 1.04 | 3.42 | 1.25 |
| Discontinued operations | 0 | 0 | 0 | 0 | (0.05) | 0.77 | (0.21) | (0.24) | (0.01) | 0.29 | 0.01 |
| 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 |
| Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | $ (695.8) | Ìý |
| Share-based Payment Arrangement [Member] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Earnings (loss) per common share attributable to Cliffs common shareholders - diluted: | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount | Ìý | Ìý | Ìý | 4.2 | Ìý | Ìý | Ìý | 3.8 | Ìý | Ìý | Ìý |
| Convertible Debt Securities [Member] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Earnings (loss) per common share attributable to Cliffs common shareholders - diluted: | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount | Ìý | Ìý | Ìý | 7.3 | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| United States [Member] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Revenues from product sales and services | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | $ 1,505.2 | 1,847.3 | $ 1,504.5 |
| Earnings (loss) per common share attributable to Cliffs common shareholders - diluted: | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | $ (460.5) | Ìý |
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- Definition Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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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 Aggregate revenue less cost of goods and services sold or operating expenses directly attributable to the revenue generation activity. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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- Definition Amount after tax of income (loss) from continuing operations attributable to the parent. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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- Definition The amount of net income (loss) from continuing operations per each share of common stock or unit outstanding during the reporting period. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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- Definition The amount of net income (loss) derived from continuing operations during 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://fasb.org/us-gaap/role/ref/legacyRef
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- Definition Amount after tax of income (loss) from a discontinued operation attributable to the parent. Includes, but is not limited to, the income (loss) from operations during the phase-out period, gain (loss) on disposal, gain (loss) for reversal of write-down (write-down) to fair value, less cost to sell, and adjustments to a prior period gain (loss) on disposal. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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- Definition Per basic share amount, after tax, of income (loss) from the day-to-day business activities of the discontinued operation and gain (loss) from the disposal of the discontinued operation. Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition Per diluted share amount, after tax, of income (loss) from the day-to-day business activities of the discontinued operation and gain (loss) from the disposal of the discontinued operation. Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition Amount, after deduction of tax, noncontrolling interests, dividends on preferred stock and participating securities; of income (loss) available to common shareholders. Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss). Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition Amount of increase (decrease) in the valuation allowance for a specified deferred tax asset. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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