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 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Beginning balance     $ (444.3)  
Beginning balance     0.2  
Beginning Balance     (444.1) $ (1,330.5)
Cumulative Effect of New Accounting Principle in Period of Adoption     34.0  
Net Income (Loss) Available to Common Stockholders, Basic     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     97.6 (2.4)
Stock and Other Incentive Plans     6.4 8.3
Undistributed Earnings Losses Related to Noncontrolling Interests     0.2 3.4
Ending balance (306.3)   (306.3)  
Ending balance 0.0   0.0  
Ending Balance (306.3) (666.7) (306.3) (666.7)
Cliffs Shareholders Equity [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Beginning balance     (444.3) (1,464.3)
Cumulative Effect of New Accounting Principle in Period of Adoption     34.0  
Other comprehensive income (loss)     16.8 1.9
TOTAL COMPREHENSIVE INCOME (LOSS) ATTRIBUTABLE TO CLIFFS SHAREHOLDERS     97.6 5.6
Stock and Other Incentive Plans     6.4 8.3
Undistributed Earnings Losses Related to Noncontrolling Interests     0.0 0.0
Ending balance (306.3) (789.1) (306.3) (789.1)
Noncontrolling Interest [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Beginning balance     0.2 133.8
Cumulative Effect of New Accounting Principle in Period of Adoption     0.0  
Net Income (Loss) Attributable to Noncontrolling Interest     0.0 (3.4)
OTHER COMPREHENSIVE LOSS (INCOME) ATTRIBUTABLE TO THE NONCONTROLLING INTEREST     0.0 (4.6)
OTHER COMPREHENSIVE LOSS (INCOME) ATTRIBUTABLE TO THE NONCONTROLLING INTEREST     0.0 (8.0)
Stock and Other Incentive Plans     0.0 0.0
Undistributed Earnings Losses Related to Noncontrolling Interests     0.2 3.4
Ending balance $ 0.0 $ 122.4 $ 0.0 122.4
Common Stock [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Stock Issued During Period, Value, New Issues       661.3
Common Stock [Member] | Cliffs Shareholders Equity [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Stock Issued During Period, Value, New Issues       661.3
Common Stock [Member] | Noncontrolling Interest [Member]        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Stock Issued During Period, Value, New Issues       $ 0.0