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]      
Related Party Transaction [Line Items]      
Foreign Currency Transaction Gain (Loss), before Tax $ 1.2 $ 13.5  
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 0.8 $ 1.5  
Liability [Member]      
Related Party Transaction [Line Items]      
Foreign Currency Transaction Gain (Loss), before Tax 2.4    
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 $ (1.7)   $ (2.4)