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)

v3.10.0.1
PROPERTY, PLANT AND EQUIPMENT (Value Of Each Of The Major Classes Of Consolidated Depreciable Assets) (Details) - USD ($)
$ in Millions
Jun. 30, 2018
Jan. 01, 2018
Dec. 31, 2017
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross $ 2,071.3   $ 2,000.7
Allowance for depreciation and depletion (990.0)   (966.9)
Property, plant and equipment, net 1,081.3 $ 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 65.1   65.8
Buildings      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 84.0   85.2
Mining equipment      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 531.2   533.9
Processing equipment      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 616.0   610.9
Electric power facilities      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 56.9   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 $ 102.2   $ 32.6