Quarterly report pursuant to Section 13 or 15(d)

SHAREHOLDERS' EQUITY (Schedule of Shareholders' Equity) (Details)

v3.8.0.1
SHAREHOLDERS' EQUITY (Schedule of Shareholders' Equity) (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Jan. 01, 2018
Dec. 31, 2017
Dec. 31, 2016
Stockholders' Equity Attributable to Parent $ (485.0)   $ (410.3) $ (444.3)  
Stockholders' Equity Attributable to Noncontrolling Interest 0.2   0.2 0.2  
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest (484.8) $ (703.0) $ (410.1) (444.1) $ (1,330.5)
Cumulative Effect of New Accounting Principle in Period of Adoption 34.0        
NET LOSS ATTRIBUTABLE TO CLIFFS SHAREHOLDERS (84.3) (28.1)      
Net Income (Loss) Attributable to Noncontrolling Interest 0.0 (1.7)      
Net Loss (84.3) (29.8)      
Other comprehensive income (loss) 7.7 (3.0)      
OTHER COMPREHENSIVE LOSS ATTRIBUTABLE TO THE NONCONTROLLING INTEREST 0.0 (5.0)      
Other Comprehensive Income (Loss), Net of Tax 7.7 (8.0)      
TOTAL COMPREHENSIVE INCOME (LOSS) ATTRIBUTABLE TO CLIFFS SHAREHOLDERS (76.6) (31.1)      
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest (76.6) (37.8)      
Stock and Other Incentive Plans 1.9 4.0      
Cliffs Shareholders Equity [Member]          
Stockholders' Equity Attributable to Parent (485.0) (830.1)   (444.3) (1,464.3)
Cumulative Effect of New Accounting Principle in Period of Adoption 34.0        
Net Income (Loss) Available to Common Stockholders, Basic (84.3) (28.1)      
Other comprehensive income (loss) 7.7 (3.0)      
TOTAL COMPREHENSIVE INCOME (LOSS) ATTRIBUTABLE TO CLIFFS SHAREHOLDERS (76.6) (31.1)      
Stock and Other Incentive Plans 1.9 4.0      
Noncontrolling Interest [Member]          
Stockholders' Equity Attributable to Noncontrolling Interest 0.2 127.1   $ 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 (1.7)      
OTHER COMPREHENSIVE LOSS ATTRIBUTABLE TO THE NONCONTROLLING INTEREST 0.0 (5.0)      
OTHER COMPREHENSIVE LOSS (INCOME) ATTRIBUTABLE TO THE NONCONTROLLING INTEREST 0.0 (6.7)      
Stock and Other Incentive Plans $ 0.0 0.0      
Common Stock [Member]          
Stock Issued During Period, Value, New Issues   661.3      
Common Stock [Member] | Cliffs Shareholders Equity [Member]          
Stock Issued During Period, Value, New Issues   661.3      
Common Stock [Member] | Noncontrolling Interest [Member]          
Stock Issued During Period, Value, New Issues   $ 0.0