Quarterly report pursuant to Section 13 or 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
Sep. 30, 2015
Dec. 31, 2014
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross $ 1,932.9 $ 1,870.2
Accumulated Depreciation and Depletion, Property, Plant and Equipment (880.3) (799.7)
Property, plant and equipment, net 1,052.6 1,070.5
Land Rights And Mineral Rights [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 500.6 500.5
Office And Information Technology [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 68.5 73.7
Buildings [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 60.1 59.8
Mining Equipment [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 583.8 585.1
Processing Equipment [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 514.4 510.2
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
Other [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 99.8 55.0
Construction in Progress [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross $ 34.5 $ 14.4