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Quarterly report pursuant to Section 13 or 15(d)

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

v3.5.0.2
BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details)
$ in Millions
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2016
USD ($)
Jun. 30, 2015
USD ($)
Jun. 30, 2016
USD ($)
Jun. 30, 2015
USD ($)
Dec. 31, 2015
USD ($)
Facility
Transaction Gains and Losses Resulting from Remeasurement [Member] Ìý Ìý Ìý Ìý Ìý
Related Party Transaction [Line Items] Ìý Ìý Ìý Ìý Ìý
Foreign Currency Transaction Gain (Loss), before Tax $ 0.2 $ (0.8) $ (0.9) $ 12.7 Ìý
Short-term intercompany loan [Member] Ìý Ìý Ìý Ìý Ìý
Related Party Transaction [Line Items] Ìý Ìý Ìý Ìý Ìý
Foreign Currency Transaction Gain (Loss), before Tax Ìý Ìý Ìý 12.4 Ìý
Cash and Cash Equivalents [Member] Ìý Ìý Ìý Ìý Ìý
Related Party Transaction [Line Items] Ìý Ìý Ìý Ìý Ìý
Foreign Currency Transaction Gain (Loss), before Tax 0.5 $ (0.7) 1.5 $ 0.7 Ìý
Liability [Member] Ìý Ìý Ìý Ìý Ìý
Related Party Transaction [Line Items] Ìý Ìý Ìý Ìý Ìý
Foreign Currency Transaction Gain (Loss), before Tax (0.2) Ìý (2.6) Ìý Ìý
Low Volatile Metallurgical Coal Mines [Member] | North American Coal [Member] Ìý Ìý Ìý Ìý Ìý
Related Party Transaction [Line Items] Ìý Ìý Ìý Ìý Ìý
Number of mines (in number of facilities) | Facility Ìý Ìý Ìý Ìý 2
Hibbing [Member] | Other Noncurrent Liabilities [Member] Ìý Ìý Ìý Ìý Ìý
Related Party Transaction [Line Items] Ìý Ìý Ìý Ìý Ìý
Equity Method Investments $ 3.7 Ìý $ 3.7 Ìý $ (2.4)