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, 2016
Dec. 31, 2015
Dec. 31, 2014
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross $ 1,941.3 $ 1,970.3  
Accumulated Depreciation and Depletion, Property, Plant and Equipment 956.9 911.3  
Property, plant and equipment, net 984.4 1,059.0 $ 1,070.5
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 65.1 71.0  
Buildings [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 67.9 60.4  
Mining Equipment [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 592.2 594.0  
Processing Equipment [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 552.0 516.8  
Electric Power Facilities [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 49.4 46.4  
Land Improvements [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 23.5 24.8  
Asset Retirement Obligation [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 19.8 87.9  
Other Assets [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 28.1 28.2  
Construction in Progress [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross $ 42.8 $ 40.3