Annual report pursuant to Section 13 and 15(d)

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

v3.8.0.1
BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details)
T in Millions
 12 Months Ended
Dec. 31, 2017
USD ($)
T
Dec. 31, 2016
USD ($)
T
Dec. 31, 2015
USD ($)
Related Party Transaction [Line Items]      
LOSS (INCOME) ATTRIBUTABLE TO NONCONTROLLING INTEREST (Year Ended December 31, 2016 - No loss related to Discontinued Operations, Year Ended December 31, 2015 - Loss of $7.7 million and Year Ended December 31, 2014 - Loss of $1,113.3 million related to Discontinued Operations) $ (3,900,000) $ 25,200,000 $ 900,000
Noncontrolling Interest, Ownership Percentage by Parent 100.00%    
LOSS ATTRIBUTABLE TO NONCONTROLLING INTEREST $ 0 0 7,700,000
Allowance for Doubtful Accounts Receivable 0 0  
Provision for Doubtful Accounts 0 0 (7,100,000)
Inventory Valuation Reserves 16,000,000 14,000,000  
Freight and venture partners' cost reimbursements $ 241,000,000 $ 195,500,000 180,900,000
Trading Day Window Determining Number of Common Shares Issuable on Conversion 20    
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests $ (105,000,000)    
U.S. Iron Ore [Member]      
Related Party Transaction [Line Items]      
Quantity Of Finished Goods | T 1.5 1.5  
Freight and venture partners' cost reimbursements $ 221,400,000 $ 174,800,000 157,300,000
Asia Pacific [Member]      
Related Party Transaction [Line Items]      
Freight and venture partners' cost reimbursements $ 19,600,000 20,700,000 $ 23,600,000
Empire [Member]      
Related Party Transaction [Line Items]      
Noncontrolling Interest, Ownership Percentage by Parent 100.00%    
Purchase of Noncontrolling Interest $ 132,700,000    
Increase in Ownership Equity 21.00%    
Stockholders' Equity, Period Increase (Decrease) $ (12,100,000)    
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests $ (116,700,000)    
Tilden [Member]      
Related Party Transaction [Line Items]      
Noncontrolling Interest, Ownership Percentage by Parent 100.00%    
Purchase of Noncontrolling Interest $ 105,000,000    
Increase in Ownership Equity 15.00%    
Stockholders' Equity, Period Increase (Decrease) $ (89,100,000)    
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests $ (15,900,000)    
Bloom Lake [Member]      
Related Party Transaction [Line Items]      
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners 17.00%    
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 $ (11,000,000) (8,700,000)  
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    
Take or Pay Contracts [Member]      
Related Party Transaction [Line Items]      
Deferred Revenue 9,600,000 3,400,000  
Customer Supplemental Payments [Member] | U.S. Iron Ore [Member]      
Related Party Transaction [Line Items]      
Deferred Revenue 64,200,000 77,100,000  
Deferred Revenue, Current 12,800,000 12,800,000  
Deferred Revenue, Noncurrent 51,400,000 $ 64,300,000  
Minimum [Member]      
Related Party Transaction [Line Items]      
Cumulative Effect of New Accounting Principle in Period of Adoption 30,000,000    
Maximum [Member]      
Related Party Transaction [Line Items]      
Cumulative Effect of New Accounting Principle in Period of Adoption $ 35,000,000    
$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