PROPERTY, PLANT AND EQUIPMENT (Value Of Each Of The Major Classes Of Consolidated Depreciable Assets) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 1,872.0	$ 1,870.2
Accumulated Depreciation and Depletion, Property, Plant and Equipment	(824.8)	(799.7)
Property, plant and equipment, net	1,047.2	1,070.5
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	69.7	73.7
Buildings [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	59.8	59.8
Mining Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	584.9	585.1
Processing Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	512.7	510.2
Electric Power Facilities [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	44.2	46.8
Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	24.8	24.7
Other [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	54.7	55.0
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 20.7	$ 14.4