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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 |

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