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DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES (Narrative) (Details) - USD ($) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2015 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Derivative [Line Items] |  |  |  |  |  |
Current Fiscal Year End Date |  |  | --12-31 |  |  |
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net |  |  | $ (6,500,000) | $ (28,200,000) | $ (51,700,000) |
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net |  |  | (18,800,000) | (31,000,000) | (22,100,000) |
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net |  |  | 5,500,000 | 161,400,000 | 141,100,000 |
Accounts receivable, net |  |  | 40,200,000 | 122,700,000 |  |
Canadian Dollar Foreign Exchange Contracts Hedge Prior to De Designation [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net |  |  | 0 | (14,300,000) | (4,100,000) |
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net |  |  | 0 |  |  |
Canadian Dollar Foreign Exchange Contracts Hedge Prior to De Designation [Member] | Cost of Sales [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net |  |  |  | (17,700,000) | (1,900,000) |
Canadian Dollar Foreign Exchange Contracts Hedge De Designated [Member] [Member] | Not Designated as Hedging Instrument [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Derivative, Notional Amount |  |  | 0 |  |  |
Foreign Exchange Contract [Member] | Other Nonoperating Income (Expense) [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Gain (Loss) on Foreign Currency Derivative Instruments Not Designated as Hedging Instruments |  |  |  |  | (600,000) |
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net | [1] |  | (3,600,000) | (16,900,000) |  |
Foreign Exchange Contract [Member] | Product Revenues [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net |  |  | (12,600,000) | 0 | 0 |
Australian Dollar Foreign Exchange Contract Hedge Designation [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net |  |  | (2,000,000) | (13,900,000) | (34,700,000) |
Australian Dollar Foreign Exchange Contract Hedge Designation [Member] | Product Revenues [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net |  | $ 6,300,000 | (7,400,000) | (13,200,000) | (11,900,000) |
Australian Dollar Foreign Exchange Contract Hedge Designation [Member] | Designated as Hedging Instrument [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Derivative, Notional Amount |  |  | 0 | 220,000,000 |  |
Australian Dollar Foreign Exchange Contract Hedge Designation [Member] | Designated as Hedging Instrument [Member] | Accumulated Other Comprehensive Income (Loss) [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Amount that will be reclassified to product revenues in the next 12 months upon settlement of the related contracts |  |  | 0 |  |  |
Australian Dollar Foreign Exchange Contracts Dedesignated [Member] | Not Designated as Hedging Instrument [Member] | Accumulated Other Comprehensive Income (Loss) [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Amount that will be reclassified to product revenues in the next 12 months upon settlement of the related contracts |  |  | 0 |  |  |
Canadian Dollar Foreign Exchange Contracts Hedge Designation [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net |  |  | 0 | 0 | (12,900,000) |
Canadian Dollar Foreign Exchange Contracts Hedge Designation [Member] | Cost of Sales [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net |  |  | 0 |  | (8,200,000) |
Interest Rate Swap [Member] | Other Nonoperating Income (Expense) [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Gain (Loss) on Fair Value Hedge Ineffectiveness, Net |  |  |  | 300,000 | 100,000 |
Interest Rate Swap [Member] | Designated as Hedging Instrument [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Derivative, Notional Amount |  |  |  | 250,000,000 |  |
Treasury Lock [Member] | Other Nonoperating Income (Expense) [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net |  |  | (100,000) | (100,000) | (100,000) |
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months |  |  | (100,000) |  |  |
Treasury Lock [Member] | Designated as Hedging Instrument [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Derivative, Notional Amount |  |  |  |  |  | $ 200,000,000
Treasury Lock [Member] | Designated as Hedging Instrument [Member] | Accumulated Other Comprehensive Income (Loss) [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net |  |  |  |  |  | $ (1,300,000)
Customer Supply Agreement [Member] | Not Designated as Hedging Instrument [Member] | Product Revenues [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Amount of gain/(loss) recognized in income on derivative |  |  | 27,100,000 | 187,800,000 | 149,200,000 |
Customer Supply Agreement [Member] | Not Designated as Hedging Instrument [Member] | Other Current Assets [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Derivative assets |  |  | 5,800,000 | 63,200,000 |  |
Provisional Pricing Arrangements [Member] | Product Revenues [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net |  |  | (1,400,000) | (9,500,000) | (7,500,000) |
Provisional Pricing Arrangements [Member] | Not Designated as Hedging Instrument [Member] | U S Iron Ore And Asia Pacific Iron Ore [Member] | Product Revenues [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Amount of gain/(loss) recognized in income on derivative |  |  | (1,400,000) | (9,500,000) | (7,500,000) |
Provisional Pricing Arrangements [Member] | Not Designated as Hedging Instrument [Member] | U S Iron Ore And Asia Pacific Iron Ore [Member] | Other Current Assets [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Derivative assets |  |  | 2,000,000 | 0 |  |
Provisional Pricing Arrangements [Member] | Not Designated as Hedging Instrument [Member] | U S Iron Ore And Asia Pacific Iron Ore [Member] | Other Current Liabilities [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Derivative liabilities |  |  | 3,400,000 | 9,500,000 |  |
$500 Million 3.95% 2018 Senior Notes [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Debt Instrument, Face Amount |  |  | 500,000,000 | 500,000,000.0 |  |
Corporate [Member] | Canadian Dollar Foreign Exchange Contracts Hedge Prior to De Designation [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net |  |  |  | (500,000) | $ (1,900,000) |
Amount that will be reclassified to product revenues in the next 12 months upon settlement of the related contracts |  |  |  | 0 |  |
Corporate [Member] | Canadian Dollar Foreign Exchange Contracts Hedge De Designated [Member] [Member] | Not Designated as Hedging Instrument [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Derivative, Notional Amount |  |  | $ 0 | 183,000,000 |  |
Corporate [Member] | Foreign Exchange Contract [Member] | Cost of Sales [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Gain (Loss) on Foreign Currency Derivative Instruments Not Designated as Hedging Instruments |  |  |  | 3,300,000 |  |
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net |  |  |  | $ (7,300,000) |  |
[1] | At December 31, 2014 and 2013, the location of the Gain (Loss) Recognized in Income on Derivative for Foreign Exchange Contracts was Cost of goods sold and operating expenses.

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