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, 2016
Mar. 31, 2015
Dec. 31, 2015
Dec. 31, 2014
Stockholders' Equity Attributable to Parent $ (1,859.4)   $ (1,981.4)  
Stockholders' Equity Attributable to Noncontrolling Interest 162.7   169.8  
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest (1,696.7) $ (1,782.1) (1,811.6) $ (1,734.3)
NET INCOME (LOSS) ATTRIBUTABLE TO CLIFFS SHAREHOLDERS 108.0 (759.8)    
INCOME (LOSS) ATTRIBUTABLE TO NONCONTROLLING INTEREST 8.8 (1.9)    
Net Income (Loss) 116.8 (761.7)    
Other Comprehensive Income (Loss), Net of Tax 6.3 196.8    
TOTAL COMPREHENSIVE INCOME (LOSS) ATTRIBUTABLE TO CLIFFS SHAREHOLDERS 113.7 (552.2)    
OTHER COMPREHENSIVE LOSS (INCOME) ATTRIBUTABLE TO THE NONCONTROLLING INTEREST 0.6 (10.8)    
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest 123.1 (564.9)    
Deconsolidation, Gain (Loss), Amount   528.2    
Stock and Other Incentive Plans 2.9 0.6    
Preferred share dividends   (12.8)    
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders (17.0)      
Undistributed Gains To Noncontrolling Interest 0.5 1.1    
Cliffs Shareholders Equity [Member]        
Stockholders' Equity Attributable to Parent (1,859.4) (1,995.7) (1,981.4) (1,431.3)
NET INCOME (LOSS) ATTRIBUTABLE TO CLIFFS SHAREHOLDERS 108.0 (759.8)    
Other comprehensive income (loss) 5.7 207.6    
TOTAL COMPREHENSIVE INCOME (LOSS) ATTRIBUTABLE TO CLIFFS SHAREHOLDERS 113.7 (552.2)    
Deconsolidation, Gain (Loss), Amount   0.0    
Stock and Other Incentive Plans 2.9 0.6    
Preferred share dividends   (12.8)    
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders 0.0      
Undistributed Gains To Noncontrolling Interest 0.0 0.0    
Noncontrolling Interest [Member]        
Stockholders' Equity Attributable to Noncontrolling Interest 162.7 213.6 $ 169.8 $ (303.0)
INCOME (LOSS) ATTRIBUTABLE TO NONCONTROLLING INTEREST 8.8 (1.9)    
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest 0.6 (10.8)    
OTHER COMPREHENSIVE LOSS (INCOME) ATTRIBUTABLE TO THE NONCONTROLLING INTEREST 9.4 (12.7)    
Deconsolidation, Gain (Loss), Amount   528.2    
Stock and Other Incentive Plans 0.0 0.0    
Preferred share dividends   0.0    
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders (17.0)      
Undistributed Gains To Noncontrolling Interest 0.5 $ 1.1    
Common Stock [Member]        
Stock Issued During Period, Value, New Issues 5.4      
Common Stock [Member] | Cliffs Shareholders Equity [Member]        
Stock Issued During Period, Value, New Issues 5.4      
Common Stock [Member] | Noncontrolling Interest [Member]        
Stock Issued During Period, Value, New Issues $ 0.0