EARNINGS PER SHARE (Earnings Per Share Computation) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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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| Earnings Per Share [Abstract] | |||||||||||
| Income from continuing operations | $ 294.5 | $ 1,039.9 | $ 360.6 | ||||||||
| Net Income (Loss) from Continuing Operations Attributable to Noncontrolling Interest | 0.0 | 0.0 | (3.9) | ||||||||
| 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 |
| Weighted average number of shares: | |||||||||||
| Basic | 276,761 | 297,176 | 288,435 | ||||||||
| Convertible senior notes | 4,400 | 3,400 | 0 | ||||||||
| Employee stock plans | 3,300 | 3,500 | 4,600 | ||||||||
| Diluted | 284,480 | 304,141 | 292,961 | ||||||||
| Earnings (loss) per common share attributable to Cliffs common shareholders - basic: | |||||||||||
| 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: | |||||||||||
| 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 |
| X | ||||||||||
- References No definition available.
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| X | ||||||||||
- 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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| X | ||||||||||
- References No definition available.
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| X | ||||||||||
- 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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- References No definition available.
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| X | ||||||||||
- 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 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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- 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 Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of convertible debt securities using the if-converted method. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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- Definition Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of share based payment arrangements using the treasury stock method. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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- Definition Amount of Net Income (Loss) attributable to noncontrolling interest. 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 The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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| X | ||||||||||
- References No definition available.
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| X | ||||||||||
- Definition Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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