PROPERTY, PLANT AND EQUIPMENT (Value Of Each Of The Major Classes Of Consolidated Depreciable Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 1,970.3	$ 1,870.2
Accumulated Depreciation and Depletion, Property, Plant and Equipment	911.3	799.7
Property, plant and equipment, net	1,059.0	1,070.5	$ 1,870.8
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	71.0	73.7
Buildings [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	60.4	59.8
Mining Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	594.0	585.1
Processing Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	516.8	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
Asset Retirement Obligation [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	87.9	26.5
Other Assets [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	28.2	28.5
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 40.3	$ 14.4