Annual report pursuant to Section 13 and 15(d)

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

v3.10.0.1
BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details)
T in Millions
 1 Months Ended 12 Months Ended
Feb. 01, 2016
Dec. 31, 2018
USD ($)
T
Dec. 31, 2017
USD ($)
T
Dec. 31, 2016
USD ($)
Jan. 01, 2018
USD ($)
Related Party Transaction [Line Items]          
Foreign Currency Transaction Gain (Loss), Realized   $ 228,100,000 $ 0 $ 0  
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net from product sales and services   422,600,000 119,300,000 57,800,000  
Contract with Customer, Liability, Revenue Recognized   51,300,000 64,200,000    
Contract with Customer, Liability, Current   21,000,000 22,400,000 16,200,000 $ 23,800,000
Deferred Revenue (Long-Term)   38,500,000 51,400,000 64,300,000  
Net Income (Loss) Attributable to Noncontrolling Interest   $ 0 (3,900,000) 25,200,000  
Noncontrolling Interest, Ownership Percentage by Parent   100.00%      
Allowance for Doubtful Accounts Receivable   $ 0 0    
Provision for Doubtful Accounts   $ 0 0    
Current Fiscal Year End Date   --12-31      
Inventory Valuation Reserves   $ 12,600,000 12,600,000    
Trading Day Window Determining Number of Common Shares Issuable on Conversion 20        
Acquisition of Noncontrolling Interest     $ (105,000,000)    
Mining and Pelletizing [Member]          
Related Party Transaction [Line Items]          
Quantity Of Finished Goods at ports | T   0.8 1.5    
Empire [Member]          
Related Party Transaction [Line Items]          
Noncontrolling Interest, Ownership Percentage by Parent     100.00%    
Distribution of Noncontrolling Interest     $ 132,700,000    
Increase in Ownership Equity     21.00%    
Stockholders' Equity, Period Increase (Decrease)     $ (12,100,000)    
Acquisition of Noncontrolling Interest     $ (116,700,000)    
Tilden [Member]          
Related Party Transaction [Line Items]          
Noncontrolling Interest, Ownership Percentage by Parent     100.00%    
Distribution of Noncontrolling Interest     $ 105,000,000    
Increase in Ownership Equity     15.00%    
Stockholders' Equity, Period Increase (Decrease)     $ (89,100,000)    
Acquisition of Noncontrolling Interest     (15,900,000)    
Investments in Ventures [Member] | Hibbing [Member]          
Related Party Transaction [Line Items]          
Ownership interest, equity method investment   23.00%      
Other Noncurrent Liabilities [Member] | Hibbing [Member]          
Related Party Transaction [Line Items]          
Equity Method Investments   $ (15,400,000) (11,000,000)    
Other Noncurrent Liabilities [Member] | Empire [Member]          
Related Party Transaction [Line Items]          
Noncontrolling Interest Purchase, Installment Amount   44,200,000      
Other Liabilities [Member] | Empire [Member]          
Related Party Transaction [Line Items]          
Noncontrolling Interest Purchase, Installment Amount   44,200,000      
Other Current Liabilities [Member]          
Related Party Transaction [Line Items]          
Contract with Customer, Liability   12,800,000 12,800,000    
Take or Pay Contracts [Member]          
Related Party Transaction [Line Items]          
Contract with Customer, Liability   $ 8,200,000 9,600,000    
$316 Million 1.5% 2025 Senior Notes [Domain]          
Related Party Transaction [Line Items]          
Debt Instrument, Interest Rate, Stated Percentage   1.50%      
Convertible debt, principal increment   $ 1,000      
$316 Million 1.5% 2025 Senior Notes [Domain] | Minimum [Member]          
Related Party Transaction [Line Items]          
Convertible debt, conversion price percentage   1.30      
$316 Million 1.5% 2025 Senior Notes [Domain] | Maximum [Member]          
Related Party Transaction [Line Items]          
Convertible debt, conversion price percentage   0.98      
Product [Member]          
Related Party Transaction [Line Items]          
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net from product sales and services   $ 422,600,000 $ 120,600,000 $ 55,900,000