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
Mar. 31, 2017
Dec. 31, 2016
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross $ 1,980.9 $ 1,941.3
Allowance for depreciation and depletion (985.9) (956.9)
Property, plant and equipment, net 995.0 984.4
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 65.8 65.1
Buildings [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 68.4 67.9
Mining Equipment [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 595.8 592.2
Processing Equipment [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 558.9 552.0
Electric Power Facilities [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 49.6 49.4
Land Improvements [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 23.7 23.5
Asset Retirement Obligation [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 19.8 19.8
Other [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 28.4 28.1
Construction in Progress [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross $ 69.8 $ 42.8