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
Jun. 30, 2017
Dec. 31, 2016
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross $ 2,006.1 $ 1,941.3
Allowance for depreciation and depletion (1,007.0) (956.9)
Property, plant and equipment, net 999.1 984.4
Land Rights And Mineral Rights [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 500.7 500.5
Office And Information Technology [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 66.0 65.1
Buildings [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 71.8 67.9
Mining Equipment [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 600.1 592.2
Processing Equipment [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 621.5 552.0
Electric Power Facilities [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 54.0 49.4
Land Improvements [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 23.8 23.5
Asset Retirement Obligation [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 19.7 19.8
Other [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 28.7 28.1
Construction in Progress [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross $ 19.8 $ 42.8