PROPERTY, PLANT AND EQUIPMENT (Value Of Each Of The Major Classes Of Consolidated Depreciable Assets) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 1,980.9	$ 1,941.3
Allowance for depreciation and depletion	(985.9)	(956.9)
Property, plant and equipment, net	995.0	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	65.8	65.1
Buildings [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	68.4	67.9
Mining Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	595.8	592.2
Processing Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	558.9	552.0
Electric Power Facilities [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	49.6	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	19.8	19.8
Other [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	28.4	28.1
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 69.8	$ 42.8