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 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Dec. 31, 2014
Dec. 31, 2013
Stockholders' Equity Attributable to Parent $ (1,923.8)   $ (1,923.8)   $ (1,431.3)  
Stockholders' Equity Attributable to Noncontrolling Interest 183.6   183.6   (303.0)  
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest (1,740.2) $ 6,829.6 (1,740.2) $ 6,829.6 (1,734.3) $ 6,884.3
NET INCOME (LOSS) ATTRIBUTABLE TO CLIFFS SHAREHOLDERS 60.2 10.9 (699.6) (59.4)    
INCOME (LOSS) ATTRIBUTABLE TO NONCONTROLLING INTEREST 5.0 3.6 3.1 3.2    
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest 65.2 14.5 (696.5) (56.2)    
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent 69.5 45.8 (482.7) 33.3    
Other Comprehensive Income (Loss), Net of Tax 10.1 35.5 206.9 93.8    
OTHER COMPREHENSIVE LOSS (INCOME) ATTRIBUTABLE TO THE NONCONTROLLING INTEREST (0.8) (0.6) 10.0 (1.1)    
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest     (489.6) 37.6    
Deconsolidation, Gain (Loss), Amount     (528.2)      
Stock and Other Incentive Plans     3.0 (3.1)    
Preferred share dividends     (12.8) (72.1)    
Undistributed Gains To Noncontrolling Interest     (34.7) (17.1)    
Cliffs Shareholders Equity [Member]            
Stockholders' Equity Attributable to Parent (1,923.8) 6,027.6 (1,923.8) 6,027.6 (1,431.3) 6,069.5
NET INCOME (LOSS) ATTRIBUTABLE TO CLIFFS SHAREHOLDERS     (699.6) (59.4)    
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent     216.9 92.7    
Comprehensive Income (Loss), Net of Tax, Attributable to Parent     (482.7) 33.3    
Deconsolidation, Gain (Loss), Amount     0.0      
Stock and Other Incentive Plans     3.0 (3.1)    
Preferred share dividends     (12.8) (72.1)    
Undistributed Gains To Noncontrolling Interest     0.0 0.0    
Noncontrolling Interest [Member]            
Stockholders' Equity Attributable to Noncontrolling Interest $ 183.6 $ 802.0 183.6 802.0 $ (303.0) $ 814.8
INCOME (LOSS) ATTRIBUTABLE TO NONCONTROLLING INTEREST     3.1 3.2    
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest     (10.0) 1.1    
OTHER COMPREHENSIVE LOSS (INCOME) ATTRIBUTABLE TO THE NONCONTROLLING INTEREST     (6.9) 4.3    
Deconsolidation, Gain (Loss), Amount     (528.2)      
Stock and Other Incentive Plans     0.0 0.0    
Preferred share dividends     0.0 0.0    
Undistributed Gains To Noncontrolling Interest     $ (34.7) $ (17.1)