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
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Dec. 31, 2015
Stockholders' Equity Attributable to Parent $ (830.1)   $ (1,464.3)  
Stockholders' Equity Attributable to Noncontrolling Interest 127.1   133.8  
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest (703.0) $ (1,696.7) (1,330.5) $ (1,811.6)
NET INCOME (LOSS) ATTRIBUTABLE TO CLIFFS SHAREHOLDERS (28.1) 108.0    
INCOME (LOSS) ATTRIBUTABLE TO NONCONTROLLING INTEREST (1.7) 8.8    
Net Income (Loss) (29.8) 116.8    
Other comprehensive income (loss) (31.1) 113.7    
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest (5.0) 0.6    
Other Comprehensive Income (Loss), Net of Tax (8.0) 6.3    
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest (37.8) 123.1    
Stock and Other Incentive Plans 4.0 2.9    
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders   (17.0)    
Undistributed Gains To Noncontrolling Interest   0.5    
Cliffs Shareholders Equity [Member]        
Stockholders' Equity Attributable to Parent (830.1) (1,859.4) (1,464.3) (1,981.4)
NET INCOME (LOSS) ATTRIBUTABLE TO CLIFFS SHAREHOLDERS (28.1) 108.0    
Other comprehensive income (loss) (3.0) 5.7    
TOTAL COMPREHENSIVE INCOME (LOSS) ATTRIBUTABLE TO CLIFFS SHAREHOLDERS (31.1) 113.7    
Stock and Other Incentive Plans 4.0 2.9    
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders   0.0    
Undistributed Gains To Noncontrolling Interest   0.0    
Noncontrolling Interest [Member]        
Stockholders' Equity Attributable to Noncontrolling Interest 127.1 162.7 $ 133.8 $ 169.8
INCOME (LOSS) ATTRIBUTABLE TO NONCONTROLLING INTEREST (1.7) 8.8    
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest (5.0) 0.6    
OTHER COMPREHENSIVE LOSS (INCOME) ATTRIBUTABLE TO THE NONCONTROLLING INTEREST (6.7) 9.4    
Stock and Other Incentive Plans 0.0 0.0    
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders   (17.0)    
Undistributed Gains To Noncontrolling Interest   (0.5)    
Common Stock [Member]        
Stock Issued During Period, Value, New Issues 661.3 5.4    
Common Stock [Member] | Cliffs Shareholders Equity [Member]        
Stock Issued During Period, Value, New Issues 661.3 5.4    
Common Stock [Member] | Noncontrolling Interest [Member]        
Stock Issued During Period, Value, New Issues $ 0.0 $ 0.0