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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 |  |

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