PROPERTY, PLANT AND EQUIPMENT (Value Of Each Of The Major Classes Of Consolidated Depreciable Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 1,941.3	$ 1,970.3
Accumulated Depreciation and Depletion, Property, Plant and Equipment	956.9	911.3
Property, plant and equipment, net	984.4	1,059.0	$ 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	65.1	71.0
Buildings [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	67.9	60.4
Mining Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	592.2	594.0
Processing Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	552.0	516.8
Electric Power Facilities [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	49.4	46.4
Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	23.5	24.8
Asset Retirement Obligation [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	19.8	87.9
Other Assets [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	28.1	28.2
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 42.8	$ 40.3