PROPERTY, PLANT AND EQUIPMENT (Value Of Each Of The Major Classes Of Consolidated Depreciable Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 2,079.4	$ 1,941.3
Accumulated Depreciation and Depletion, Property, Plant and Equipment	1,028.4	956.9
Property, plant and equipment, net	1,051.0	984.4	$ 1,059.0
Land Rights And Mineral Rights [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	549.6	500.5
Office And Information Technology [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	66.3	65.1
Buildings [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	86.8	67.9
Mining Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	594.4	592.2
Processing Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	617.0	552.0
Electric Power Facilities [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	57.0	49.4
Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	23.7	23.5
Asset Retirement Obligation [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	19.2	19.8
Other Assets [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	30.3	28.1
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 35.1	$ 42.8