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Quarterly report pursuant to Section 13 or 15(d)

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

v3.4.0.3
BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2016
USD ($)
Mar. 31, 2015
USD ($)
Dec. 31, 2015
USD ($)
Facility
Transaction Gains and Losses Resulting from Remeasurement [Member] Ìý Ìý Ìý
Related Party Transaction [Line Items] Ìý Ìý Ìý
Foreign Currency Transaction Gain (Loss), before Tax $ 1.2 $ 13.5 Ìý
Short-term intercompany loan [Member] Ìý Ìý Ìý
Related Party Transaction [Line Items] Ìý Ìý Ìý
Foreign Currency Transaction Gain (Loss), before Tax Ìý 12.4 Ìý
Cash and Cash Equivalents [Member] Ìý Ìý Ìý
Related Party Transaction [Line Items] Ìý Ìý Ìý
Foreign Currency Transaction Gain (Loss), before Tax 0.8 $ 1.5 Ìý
Liability [Member] Ìý Ìý Ìý
Related Party Transaction [Line Items] Ìý Ìý Ìý
Foreign Currency Transaction Gain (Loss), before Tax 2.4 Ìý Ìý
Low Volatile Metallurgical Coal Mines [Member] | North American Coal [Member] Ìý Ìý Ìý
Related Party Transaction [Line Items] Ìý Ìý Ìý
Number of mines (in number of facilities) | Facility Ìý Ìý 2
Hibbing [Member] | Other Noncurrent Liabilities [Member] Ìý Ìý Ìý
Related Party Transaction [Line Items] Ìý Ìý Ìý
Equity Method Investments $ (1.7) Ìý $ (2.4)