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, 2016
Dec. 31, 2015
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross $ 1,947.0 $ 1,970.3
Allowance for depreciation and depletion (937.4) (911.3)
Property, plant and equipment, net 1,009.6 1,059.0
Land Rights And Mineral Rights [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 500.5 500.5
Office And Information Technology [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 57.9 71.0
Buildings [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 60.5 60.4
Mining Equipment [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 597.4 594.0
Processing Equipment [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 519.4 516.8
Electric Power Facilities [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 47.3 46.4
Land Improvements [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 24.8 24.8
Asset Retirement Obligation [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 67.1 87.9
Other [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 29.2 28.2
Construction in Progress [Member]    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross $ 42.9 $ 40.3