PROPERTY, PLANT AND EQUIPMENT (Value Of Each Of The Major Classes Of Consolidated Depreciable Assets) (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 1,947.0	$ 1,970.3
Allowance for depreciation and depletion	(937.4)	(911.3)
Property, plant and equipment, net	1,009.6	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	57.9	71.0
Buildings [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	60.5	60.4
Mining Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	597.4	594.0
Processing Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	519.4	516.8
Electric Power Facilities [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	47.3	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	67.1	87.9
Other [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	29.2	28.2
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 42.9	$ 40.3