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, 2018
Jan. 01, 2018
Dec. 31, 2017
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross $ 2,308.4   $ 2,000.7
Accumulated Depreciation and Depletion, Property, Plant and Equipment (1,022.4)   (966.9)
Property, plant and equipment, net 1,286.0 $ 1,033.8 1,033.8
Land Rights And Mineral Rights [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 549.6   549.6
Office And Information Technology [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 70.0   65.8
Buildings [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 87.2   85.2
Mining Equipment [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 548.5   533.9
Processing Equipment [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 645.8   610.9
Electric Power Facilities [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 58.7   56.9
Land Improvements [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 23.8   23.7
Asset Retirement Obligation [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 14.8   16.9
Other Assets [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 25.2   25.2
Construction in Progress [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross $ 284.8   $ 32.6