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)

v3.8.0.1
PROPERTY, PLANT AND EQUIPMENT (Value Of Each Of The Major Classes Of Consolidated Depreciable Assets) (Details) - USD ($)
$ in Millions
Mar. 31, 2018
Jan. 01, 2018
Dec. 31, 2017
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross $ 2,091.1   $ 2,079.4
Allowance for depreciation and depletion (1,043.8)   (1,028.4)
Property, plant and equipment, net 1,047.3 $ 1,051.0 1,051.0
Land Rights And Mineral Rights [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 549.6   549.6
Office And Information Technology [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 66.3   66.3
Buildings [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 85.5   86.8
Mining Equipment [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 594.0   594.4
Processing Equipment [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 619.8   617.0
Electric Power Facilities [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 57.0   57.0
Land Improvements [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 23.6   23.7
Asset Retirement Obligation [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 16.9   19.2
Other [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 30.3   30.3
Construction in Progress [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross $ 48.1   $ 35.1