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]          
Related Party Transaction [Line Items]          
Foreign Currency Transaction Gain (Loss), before Tax $ 0.2 $ (0.8) $ (0.9) $ 12.7  
Short-term intercompany loan [Member]          
Related Party Transaction [Line Items]          
Foreign Currency Transaction Gain (Loss), before Tax       12.4  
Cash and Cash Equivalents [Member]          
Related Party Transaction [Line Items]          
Foreign Currency Transaction Gain (Loss), before Tax 0.5 $ (0.7) 1.5 $ 0.7  
Liability [Member]          
Related Party Transaction [Line Items]          
Foreign Currency Transaction Gain (Loss), before Tax (0.2)   (2.6)    
Low Volatile Metallurgical Coal Mines [Member] | North American Coal [Member]          
Related Party Transaction [Line Items]          
Number of mines (in number of facilities) | Facility         2
Hibbing [Member] | Other Noncurrent Liabilities [Member]          
Related Party Transaction [Line Items]          
Equity Method Investments $ 3.7   $ 3.7   $ (2.4)