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, 2017
Dec. 31, 2016
Dec. 31, 2015
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross $ 2,079.4 $ 1,941.3  
Accumulated Depreciation and Depletion, Property, Plant and Equipment 1,028.4 956.9  
Property, plant and equipment, net 1,051.0 984.4 $ 1,059.0
Land Rights And Mineral Rights [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 549.6 500.5  
Office And Information Technology [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 66.3 65.1  
Buildings [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 86.8 67.9  
Mining Equipment [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 594.4 592.2  
Processing Equipment [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 617.0 552.0  
Electric Power Facilities [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 57.0 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 19.2 19.8  
Other Assets [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 30.3 28.1  
Construction in Progress [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross $ 35.1 $ 42.8