PROPERTY, PLANT AND EQUIPMENT (Value Of Each Of The Major Classes Of Consolidated Depreciable Assets) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 1,924.1	$ 1,970.3
Allowance for depreciation and depletion	(934.0)	(911.3)
Property, plant and equipment, net	990.1	1,059.0
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	63.6	71.0
Buildings [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	61.1	60.4
Mining Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	598.7	594.0
Processing Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	541.2	516.8
Electric Power Facilities [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	49.6	46.4
Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	24.8	24.8
Asset Retirement Obligation [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	18.3	87.9
Other [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	28.4	28.2
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 37.9	$ 40.3