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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 9 Months Ended 12 Months Ended
Sep. 30, 2016
USD ($)
Sep. 30, 2015
USD ($)
Sep. 30, 2016
USD ($)
Sep. 30, 2015
USD ($)
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 $ (0.3) $ 2.4 $ (1.2) $ 15.2 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 0.1 Ìý 11.1 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 (1.1) $ 1.3 0.3 $ 2.0 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 0.6 Ìý (2.0) 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 $ (1.9) Ìý $ (1.9) Ìý $ (2.4)