Annual report pursuant to Section 13 and 15(d)

PROPERTY, PLANT AND EQUIPMENT (Value Of Each Of The Major Classes Of Consolidated Depreciable Assets) (Details)

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PROPERTY, PLANT AND EQUIPMENT (Value Of Each Of The Major Classes Of Consolidated Depreciable Assets) (Details) - USD ($)
$ in Millions
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross $ 1,970.3 $ 1,870.2  
Accumulated Depreciation and Depletion, Property, Plant and Equipment 911.3 799.7  
Property, plant and equipment, net 1,059.0 1,070.5 $ 1,870.8
Land Rights And Mineral Rights [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 500.5 500.5  
Office And Information Technology [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 71.0 73.7  
Buildings [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 60.4 59.8  
Mining Equipment [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 594.0 585.1  
Processing Equipment [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 516.8 510.2  
Electric Power Facilities [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 46.4 46.8  
Land Improvements [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 24.8 24.7  
Asset Retirement Obligation [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 87.9 26.5  
Other Assets [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 28.2 28.5  
Construction in Progress [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross $ 40.3 $ 14.4