QUARTERLY RESULTS OF OPERATIONS (Schedule of Quarterly Financial Information) (Details) - USD ($) $ / shares in Units, $ in Millions |
3 Months Ended | 12 Months Ended | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
Dec. 31, 2018 |
Sep. 30, 2018 |
Jun. 30, 2018 |
Mar. 31, 2018 |
Dec. 31, 2017 |
Sep. 30, 2017 |
Jun. 30, 2017 |
Mar. 31, 2017 |
Dec. 31, 2018 |
Dec. 31, 2017 |
Dec. 31, 2016 |
|
| Revenues | $ 696.3 | $ 741.8 | $ 714.3 | $ 180.0 | $ 511.8 | $ 596.7 | $ 471.3 | $ 286.2 | $ 2,332.4 | $ 1,866.0 | $ 1,554.5 |
| SALES MARGIN | 202.0 | 261.6 | 284.5 | 61.5 | 116.1 | 157.8 | 144.7 | 49.0 | 809.6 | 467.6 | 280.1 |
| Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest | 333.0 | 22.3 | 83.8 | (78.5) | 1,039.9 | 360.6 | 122.6 | ||||
| Income (Loss) from Continuing Operations, Net of Tax, Attributable to Noncontrolling Interest | 0.0 | 0.5 | 1.7 | 1.7 | |||||||
| Income (Loss) from Continuing Operations Attributable to Parent | 624.1 | 199.8 | 229.4 | (13.4) | 333.0 | 22.8 | 85.5 | (76.8) | 1,039.9 | 364.5 | 97.4 |
| INCOME FROM DISCONTINUED OPERATIONS, net of tax | (14.6) | 238.0 | (64.3) | (70.9) | (23.1) | 30.6 | (53.7) | 48.7 | 88.2 | 2.5 | 76.7 |
| Net income attributable to Cliffs shareholders | $ 609.5 | $ 437.8 | $ 165.1 | $ (84.3) | $ 309.9 | $ 53.4 | $ 31.8 | $ (28.1) | $ 1,128.1 | $ 367.0 | $ 174.1 |
| Continuing operations | $ 2.11 | $ 0.67 | $ 0.77 | $ (0.05) | $ 1.12 | $ 0.08 | $ 0.28 | $ (0.29) | $ 3.50 | $ 1.27 | $ 0.49 |
| Discontinued operations | (0.05) | 0.80 | (0.22) | (0.24) | (0.08) | 0.10 | (0.18) | 0.18 | 0.30 | 0.01 | 0.39 |
| Earnings per common share attributable to Cliffs common shareholders - basic: | 2.06 | 1.47 | 0.55 | (0.29) | 1.04 | 0.18 | 0.10 | (0.11) | 3.80 | 1.28 | 0.88 |
| Continuing operations | 2.03 | 0.64 | 0.76 | (0.05) | 1.11 | 0.08 | 0.28 | (0.29) | 3.42 | 1.25 | 0.49 |
| Discontinued operations | (0.05) | 0.77 | (0.21) | (0.24) | (0.08) | 0.10 | (0.18) | 0.18 | 0.29 | 0.01 | 0.38 |
| Earnings per common share attributable to Cliffs common shareholders - diluted: | $ 1.98 | $ 1.41 | $ 0.55 | $ (0.29) | $ 1.03 | $ 0.18 | $ 0.10 | $ (0.11) | $ 3.71 | $ 1.26 | $ 0.87 |
| Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount | 3,800,000.0 | 4,600,000.0 | 0 | ||||||||
| X | ||||||||||
- 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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| X | ||||||||||
- Definition Amount after tax of income (loss) from continuing operations attributable to the noncontrolling interest. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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| X | ||||||||||
- Definition Amount after tax of income (loss) from continuing operations including portion attributable to the noncontrolling interest. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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| X | ||||||||||
- 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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| X | ||||||||||
- 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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| X | ||||||||||
- 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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| X | ||||||||||
- 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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| X | ||||||||||
- Definition The portion of profit or loss for the period, net of income taxes, which is attributable to the parent. 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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