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 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Dec. 31, 2015
Stockholders' Equity Attributable to Parent $ (833.3)   $ (833.3)   $ (1,464.3)  
Stockholders' Equity Attributable to Noncontrolling Interest 0.2   0.2   133.8  
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest (833.1) $ (1,400.5) (833.1) $ (1,400.5) (1,330.5) $ (1,811.6)
NET INCOME (LOSS) ATTRIBUTABLE TO CLIFFS SHAREHOLDERS 53.4 (25.8) 57.1 95.0    
Net Income (Loss) Attributable to Noncontrolling Interest (0.5) (2.0) (3.9) 23.5    
Net Income (Loss) 52.9 (27.8) 53.2 118.5    
Other comprehensive income (loss) 55.7 (18.0) 61.3 111.8    
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest 5.7 0.9 1.1 2.2    
Other Comprehensive Income (Loss), Net of Tax 8.0 8.7 5.3 19.0    
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest     58.5 137.5    
Stock and Other Incentive Plans     13.5 10.1    
Distributions to partnership equity       (48.8)    
Distributions to noncontrolling interest     1.8 (2.9)    
Cliffs Shareholders Equity [Member]            
Stockholders' Equity Attributable to Parent (833.3) (1,544.3) (833.3) (1,544.3) (1,464.3) (1,981.4)
Net Income (Loss) Available to Common Stockholders, Basic     57.1 95.0    
Other comprehensive income (loss)     4.2 16.8    
TOTAL COMPREHENSIVE INCOME (LOSS) ATTRIBUTABLE TO CLIFFS SHAREHOLDERS     61.3 111.8    
Stock and Other Incentive Plans     13.5 10.1    
Distributions to partnership equity       0.0    
Distributions to noncontrolling interest     0.0 0.0    
Noncontrolling Interest [Member]            
Stockholders' Equity Attributable to Noncontrolling Interest 0.2 $ 143.8 0.2 143.8 $ 133.8 $ 169.8
Net Income (Loss) Attributable to Noncontrolling Interest     (3.9) 23.5    
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest     1.1 2.2    
OTHER COMPREHENSIVE LOSS (INCOME) ATTRIBUTABLE TO THE NONCONTROLLING INTEREST     (2.8) 25.7    
Stock and Other Incentive Plans     0.0 0.0    
Distributions to partnership equity       (48.8)    
Distributions to noncontrolling interest     1.8 (2.9)    
Common Stock [Member]            
Stock Issued During Period, Value, New Issues     661.3 315.2    
Common Stock [Member] | Cliffs Shareholders Equity [Member]            
Stock Issued During Period, Value, New Issues     661.3 315.2    
Common Stock [Member] | Noncontrolling Interest [Member]            
Stock Issued During Period, Value, New Issues     0.0 $ 0.0    
Tilden [Member]            
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests 15.9   (105.0)      
Tilden [Member] | Cliffs Shareholders Equity [Member]            
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests     (89.1)      
Tilden [Member] | Noncontrolling Interest [Member]            
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests     (15.9)      
Empire [Member]            
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests $ 116.7   (132.7)      
Empire [Member] | Cliffs Shareholders Equity [Member]            
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests     (16.0)      
Empire [Member] | Noncontrolling Interest [Member]            
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests     $ (116.7)