Annual report pursuant to Section 13 and 15(d)

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

v3.6.0.2
BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details)
T in Millions
 12 Months Ended
Dec. 31, 2016
USD ($)
T
Dec. 31, 2015
USD ($)
T
Dec. 31, 2014
USD ($)
Related Party Transaction [Line Items]      
Net Income (Loss) Attributable to Noncontrolling Interest $ 25,200,000 $ 900,000 $ (1,087,400,000)
LOSS ATTRIBUTABLE TO NONCONTROLLING INTEREST 0 7,700,000 1,113,300,000
Allowance for Doubtful Accounts Receivable 0 7,100,000  
Provision for Doubtful Accounts 0 7,100,000.0 0
Inventory Valuation Reserves 14,000,000 31,800,000  
Disposal of supplies inventory 17,400,000    
Tangible Asset Impairment Charges 0 0 537,800,000
Impairment of Intangible Assets (Excluding Goodwill)     13,800,000
Freight and venture partners' cost reimbursements $ 195,500,000 $ 180,900,000 278,000,000
Trading Day Window Determining Number of Common Shares Issuable on Conversion 20    
U.S. Iron Ore [Member]      
Related Party Transaction [Line Items]      
Quantity Of Finished Goods | T 1.5 1.3  
Freight and venture partners' cost reimbursements $ 174,800,000 $ 157,300,000 271,000,000
Asia Pacific [Member]      
Related Party Transaction [Line Items]      
Freight and venture partners' cost reimbursements $ 20,700,000 23,600,000 6,900,000
Empire [Member]      
Related Party Transaction [Line Items]      
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners 21.00%    
Net Income (Loss) Attributable to Noncontrolling Interest   (8,600,000)  
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 $ (8,700,000) (2,400,000)  
Take or Pay Contracts [Member]      
Related Party Transaction [Line Items]      
Deferred Revenue 3,400,000   $ 29,300,000
Customer Supplemental Payments [Member] | U.S. Iron Ore [Member]      
Related Party Transaction [Line Items]      
Deferred Revenue 77,100,000 89,900,000  
Deferred Revenue, Current 16,400,000 12,800,000  
Deferred Revenue, Noncurrent $ 64,200,000 $ 77,100,000