PROPERTY, PLANT AND EQUIPMENT (Value Of Each Of The Major Classes Of Consolidated Depreciable Assets) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 1,932.9	$ 1,870.2
Accumulated Depreciation and Depletion, Property, Plant and Equipment	(880.3)	(799.7)
Property, plant and equipment, net	1,052.6	1,070.5
Land Rights And Mineral Rights [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	500.6	500.5
Office And Information Technology [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	68.5	73.7
Buildings [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	60.1	59.8
Mining Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	583.8	585.1
Processing Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	514.4	510.2
Electric Power Facilities [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	46.4	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	99.8	55.0
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 34.5	$ 14.4