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Quarterly report pursuant to Section 13 or 15(d)

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

v3.10.0.1
SHAREHOLDERS' EQUITY (Schedule of Shareholders' Equity) (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Sep. 30, 2018
Sep. 30, 2017
NET INCOME ATTRIBUTABLE TO CLIFFS SHAREHOLDERS $ 437.8 Ìý Ìý $ 53.4 Ìý Ìý $ 518.6 $ 57.1
Increase (Decrease) in Stockholders' Equity [Roll Forward] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Beginning balance Ìý Ìý $ (444.3) Ìý Ìý Ìý (444.3) Ìý
Beginning balance Ìý Ìý 0.2 Ìý Ìý Ìý 0.2 Ìý
Beginning Balance (306.3) $ (484.8) (444.1) (666.7) $ (703.0) $ (1,330.5) (444.1) (1,330.5)
Cumulative Effect of New Accounting Principle in Period of Adoption Ìý Ìý 34.0 Ìý Ìý Ìý Ìý Ìý
Net Income (Loss) Available to Common Stockholders, Basic Ìý 165.1 (84.3) 53.4 31.8 (28.1) Ìý Ìý
Net Income (Loss) Attributable to Noncontrolling Interest 0.0 Ìý Ìý (0.5) Ìý Ìý 0.0 (3.9)
NET INCOME 437.8 165.1 (84.3) 52.9 30.1 (29.8) 518.6 53.2
Other comprehensive income (loss) (221.2) Ìý Ìý 2.3 Ìý Ìý (204.4) 4.2
OTHER COMPREHENSIVE INCOME ATTRIBUTABLE TO THE NONCONTROLLING INTEREST 0.0 Ìý Ìý 5.7 Ìý Ìý 0.0 1.1
Other Comprehensive Income (Loss), Net of Tax (221.2) 9.1 7.7 8.0 5.3 (8.0) (204.4) 5.3
TOTAL COMPREHENSIVE INCOME ATTRIBUTABLE TO CLIFFS SHAREHOLDERS 216.6 Ìý Ìý 55.7 Ìý Ìý 314.2 61.3
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest 216.6 174.2 (76.6) 60.9 35.4 (37.8) Ìý Ìý
Stock and Other Incentive Plans 3.5 4.5 1.9 5.2 4.3 4.0 Ìý Ìý
Purchase of Noncontrolling Interest Ìý Ìý Ìý (105.0) Ìý Ìý Ìý Ìý
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders Ìý Ìý Ìý (132.7) Ìý Ìý Ìý Ìý
Undistributed Earnings Losses Related to Noncontrolling Interests Ìý (0.2) Ìý 5.2 (3.4) Ìý Ìý Ìý
Ending balance (86.2) Ìý Ìý Ìý Ìý Ìý (86.2) Ìý
Ending balance 0.0 Ìý Ìý Ìý Ìý Ìý 0.0 Ìý
Ending Balance (86.2) (306.3) (484.8) (833.1) (666.7) (703.0) (86.2) (833.1)
Cliffs Shareholders Equity [Member] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Increase (Decrease) in Stockholders' Equity [Roll Forward] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Beginning balance (306.3) (485.0) (444.3) (789.1) (830.1) (1,464.3) (444.3) (1,464.3)
Cumulative Effect of New Accounting Principle in Period of Adoption Ìý Ìý 34.0 Ìý Ìý Ìý Ìý Ìý
Other comprehensive income (loss) (221.2) 9.1 7.7 2.3 4.9 (3.0) Ìý Ìý
TOTAL COMPREHENSIVE INCOME ATTRIBUTABLE TO CLIFFS SHAREHOLDERS 216.6 174.2 (76.6) 55.7 36.7 (31.1) Ìý Ìý
Stock and Other Incentive Plans 3.5 4.5 1.9 5.2 4.3 4.0 Ìý Ìý
Purchase of Noncontrolling Interest Ìý Ìý Ìý (89.1) Ìý Ìý Ìý Ìý
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders Ìý Ìý Ìý 16.0 Ìý Ìý Ìý Ìý
Undistributed Earnings Losses Related to Noncontrolling Interests Ìý 0.0 Ìý 0.0 0.0 Ìý Ìý Ìý
Ending balance (86.2) (306.3) (485.0) (833.3) (789.1) (830.1) (86.2) (833.3)
Noncontrolling Interest [Member] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Increase (Decrease) in Stockholders' Equity [Roll Forward] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Beginning balance 0.0 0.2 0.2 122.4 127.1 133.8 0.2 133.8
Cumulative Effect of New Accounting Principle in Period of Adoption Ìý Ìý 0.0 Ìý Ìý Ìý Ìý Ìý
Net Income (Loss) Attributable to Noncontrolling Interest 0.0 0.0 0.0 (0.5) (1.7) (1.7) Ìý Ìý
OTHER COMPREHENSIVE INCOME ATTRIBUTABLE TO THE NONCONTROLLING INTEREST 0.0 0.0 0.0 5.7 0.4 (5.0) Ìý Ìý
OTHER COMPREHENSIVE LOSS (INCOME) ATTRIBUTABLE TO THE NONCONTROLLING INTEREST 0.0 0.0 0.0 5.2 (1.3) (6.7) Ìý Ìý
Stock and Other Incentive Plans 0.0 0.0 0.0 0.0 0.0 0.0 Ìý Ìý
Purchase of Noncontrolling Interest Ìý Ìý Ìý (15.9) Ìý Ìý Ìý Ìý
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders Ìý Ìý Ìý 116.7 Ìý Ìý Ìý Ìý
Undistributed Earnings Losses Related to Noncontrolling Interests Ìý 0.2 Ìý (5.2) (3.4) Ìý Ìý Ìý
Ending balance $ 0.0 $ 0.0 $ 0.2 $ 0.2 $ 122.4 127.1 0.0 $ 0.2
Common Stock [Member] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Increase (Decrease) in Stockholders' Equity [Roll Forward] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Stock Issued During Period, Value, New Issues Ìý Ìý Ìý Ìý Ìý 661.3 Ìý Ìý
Common Stock [Member] | Cliffs Shareholders Equity [Member] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Increase (Decrease) in Stockholders' Equity [Roll Forward] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Stock Issued During Period, Value, New Issues Ìý Ìý Ìý Ìý Ìý 661.3 Ìý Ìý
Common Stock [Member] | Noncontrolling Interest [Member] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Increase (Decrease) in Stockholders' Equity [Roll Forward] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Stock Issued During Period, Value, New Issues Ìý Ìý Ìý Ìý Ìý $ 0.0 Ìý Ìý
Empire Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Increase (Decrease) in Stockholders' Equity [Roll Forward] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Purchase of Noncontrolling Interest Ìý Ìý Ìý Ìý Ìý Ìý $ 132.7 Ìý