Annual report pursuant to Section 13 and 15(d)

BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details)

v3.19.3.a.u2
BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details)
T in Millions, $ in Millions
3 Months Ended 12 Months Ended
Dec. 31, 2019
USD ($)
T
Sep. 30, 2019
USD ($)
Jun. 30, 2019
USD ($)
Mar. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
T
Sep. 30, 2018
USD ($)
Jun. 30, 2018
USD ($)
Mar. 31, 2018
USD ($)
Dec. 31, 2019
USD ($)
unit
T
Dec. 31, 2018
USD ($)
T
Dec. 31, 2017
USD ($)
T
Jan. 01, 2018
USD ($)
Dec. 31, 2016
USD ($)
Related Party Transaction [Line Items]                          
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest $ 357.9       $ 424.2       $ 357.9 $ 424.2 $ (444.1)   $ (1,330.5)
Number of Reporting Units | unit                 2        
Noncontrolling Interest, Ownership Percentage by Parent 100.00%               100.00%        
Net Income (Loss) Attributable to Noncontrolling Interest                 $ 0.0 0.0 (3.9)    
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net from product sales and services                 7.5 422.6 119.3    
Foreign Currency Transaction Gain (Loss), Realized                 0.0 228.1 0.0    
Revenues $ 534.1 $ 555.6 $ 743.2 $ 157.0 $ 696.3 $ 741.8 $ 714.3 $ 180.0 1,989.9 2,332.4 1,866.0    
Net income                 $ 292.8 $ 1,128.1 $ 363.1    
Hibbing [Member]                          
Related Party Transaction [Line Items]                          
Ownership interest, equity method investment 23.00%               23.00%        
Mining and Pelletizing [Member]                          
Related Party Transaction [Line Items]                          
Quantity Of Finished Goods at ports | T 1.0       0.8       1.0 0.8 1.5    
Revenues                 $ 1,989.9 $ 2,332.4 $ 1,866.0    
Empire [Member]                          
Related Party Transaction [Line Items]                          
Distribution of Noncontrolling Interest                     132.7    
Stockholders' Equity, Period Increase (Decrease)                     (12.1)    
Net Income (Loss) Attributable to Noncontrolling Interest                     (3.9)    
Acquisition of Noncontrolling Interest                     (116.7)    
Tilden [Member]                          
Related Party Transaction [Line Items]                          
Distribution of Noncontrolling Interest                     105.0    
Stockholders' Equity, Period Increase (Decrease)                     (89.1)    
Acquisition of Noncontrolling Interest                     $ (15.9)    
Other Noncurrent Liabilities [Member] | Hibbing [Member]                          
Related Party Transaction [Line Items]                          
Equity Method Investments $ (18.0)       $ (15.4)       $ (18.0) (15.4)      
Other Current Liabilities [Member] | Empire [Member]                          
Related Party Transaction [Line Items]                          
Noncontrolling Interest Purchase, Installment Amount         $ 44.2         44.2      
Tilden [Member]                          
Related Party Transaction [Line Items]                          
Noncontrolling Interest, Ownership Percentage by Parent 100.00%               100.00%        
Increase in Ownership Equity                     15.00%    
Empire [Member]                          
Related Party Transaction [Line Items]                          
Noncontrolling Interest, Ownership Percentage by Parent 100.00%               100.00%        
Increase in Ownership Equity                     21.00%    
Difference between Revenue Guidance in Effect before and after Topic 606 [Member]                          
Related Party Transaction [Line Items]                          
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest                       $ 34.0  
Revenues                   68.1      
Net income                   $ 46.2      
Sales [Member]                          
Related Party Transaction [Line Items]                          
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net from product sales and services                     $ 120.6