PROPERTY, PLANT AND EQUIPMENT (Value Of Each Of The Major Classes Of Consolidated Depreciable Assets) (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 2,006.1	$ 1,941.3
Allowance for depreciation and depletion	(1,007.0)	(956.9)
Property, plant and equipment, net	999.1	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	66.0	65.1
Buildings [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	71.8	67.9
Mining Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	600.1	592.2
Processing Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	621.5	552.0
Electric Power Facilities [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	54.0	49.4
Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	23.8	23.5
Asset Retirement Obligation [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	19.7	19.8
Other [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	28.7	28.1
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 19.8	$ 42.8