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, 2018
Jan. 01, 2018
Dec. 31, 2017
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross $ 2,152.9   $ 2,000.7
Allowance for depreciation and depletion (1,008.1)   (966.9)
Property, plant and equipment, net 1,144.8 $ 1,033.8 1,033.8
Land rights and mineral rights      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 549.6   549.6
Office and information technology      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 67.8   65.8
Buildings      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 84.1   85.2
Mining equipment      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 538.7   533.9
Processing equipment      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 619.0   610.9
Electric power facilities      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 58.7   56.9
Land improvements      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 24.2   23.7
Asset retirement obligation      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 16.9   16.9
Other      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 25.2   25.2
Construction in-progress      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross $ 168.7   $ 32.6