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Quarterly report pursuant to Section 13 or 15(d)

SHAREHOLDERS' EQUITY (Schedule of Shareholders' Equity) (Details)

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SHAREHOLDERS' EQUITY (Schedule of Shareholders' Equity) (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Increase (Decrease) in Stockholders' Equity [Roll Forward] Ìý Ìý Ìý Ìý
Beginning balance Ìý Ìý $ (444.3) Ìý
Beginning balance Ìý Ìý 0.2 Ìý
Beginning Balance Ìý Ìý (444.1) $ (1,330.5)
Cumulative Effect of New Accounting Principle in Period of Adoption Ìý Ìý 34.0 Ìý
Net Income (Loss) Available to Common Stockholders, Basic Ìý Ìý 80.8 3.7
Net Income (Loss) Attributable to Noncontrolling Interest $ 0.0 $ (1.7) 0.0 (3.4)
NET INCOME 165.1 30.1 80.8 0.3
Other comprehensive income (loss) 9.1 4.9 16.8 1.9
OTHER COMPREHENSIVE LOSS (INCOME) ATTRIBUTABLE TO THE NONCONTROLLING INTEREST 0.0 0.4 0.0 (4.6)
Other Comprehensive Income (Loss), Net of Tax 9.1 5.3 16.8 (2.7)
TOTAL COMPREHENSIVE INCOME (LOSS) ATTRIBUTABLE TO CLIFFS SHAREHOLDERS 174.2 36.7 97.6 5.6
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Ìý Ìý 97.6 (2.4)
Stock and Other Incentive Plans Ìý Ìý 6.4 8.3
Undistributed Earnings Losses Related to Noncontrolling Interests Ìý Ìý 0.2 3.4
Ending balance (306.3) Ìý (306.3) Ìý
Ending balance 0.0 Ìý 0.0 Ìý
Ending Balance (306.3) (666.7) (306.3) (666.7)
Cliffs Shareholders Equity [Member] Ìý Ìý Ìý Ìý
Increase (Decrease) in Stockholders' Equity [Roll Forward] Ìý Ìý Ìý Ìý
Beginning balance Ìý Ìý (444.3) (1,464.3)
Cumulative Effect of New Accounting Principle in Period of Adoption Ìý Ìý 34.0 Ìý
Other comprehensive income (loss) Ìý Ìý 16.8 1.9
TOTAL COMPREHENSIVE INCOME (LOSS) ATTRIBUTABLE TO CLIFFS SHAREHOLDERS Ìý Ìý 97.6 5.6
Stock and Other Incentive Plans Ìý Ìý 6.4 8.3
Undistributed Earnings Losses Related to Noncontrolling Interests Ìý Ìý 0.0 0.0
Ending balance (306.3) (789.1) (306.3) (789.1)
Noncontrolling Interest [Member] Ìý Ìý Ìý Ìý
Increase (Decrease) in Stockholders' Equity [Roll Forward] Ìý Ìý Ìý Ìý
Beginning balance Ìý Ìý 0.2 133.8
Cumulative Effect of New Accounting Principle in Period of Adoption Ìý Ìý 0.0 Ìý
Net Income (Loss) Attributable to Noncontrolling Interest Ìý Ìý 0.0 (3.4)
OTHER COMPREHENSIVE LOSS (INCOME) ATTRIBUTABLE TO THE NONCONTROLLING INTEREST Ìý Ìý 0.0 (4.6)
OTHER COMPREHENSIVE LOSS (INCOME) ATTRIBUTABLE TO THE NONCONTROLLING INTEREST Ìý Ìý 0.0 (8.0)
Stock and Other Incentive Plans Ìý Ìý 0.0 0.0
Undistributed Earnings Losses Related to Noncontrolling Interests Ìý Ìý 0.2 3.4
Ending balance $ 0.0 $ 122.4 $ 0.0 122.4
Common Stock [Member] Ìý Ìý Ìý Ìý
Increase (Decrease) in Stockholders' Equity [Roll Forward] Ìý Ìý Ìý Ìý
Stock Issued During Period, Value, New Issues Ìý Ìý Ìý 661.3
Common Stock [Member] | Cliffs Shareholders Equity [Member] Ìý Ìý Ìý Ìý
Increase (Decrease) in Stockholders' Equity [Roll Forward] Ìý Ìý Ìý Ìý
Stock Issued During Period, Value, New Issues Ìý Ìý Ìý 661.3
Common Stock [Member] | Noncontrolling Interest [Member] Ìý Ìý Ìý Ìý
Increase (Decrease) in Stockholders' Equity [Roll Forward] Ìý Ìý Ìý Ìý
Stock Issued During Period, Value, New Issues Ìý Ìý Ìý $ 0.0