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