PROPERTY, PLANT AND EQUIPMENT (Value Of Each Of The Major Classes Of Consolidated Depreciable Assets) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 2,091.1		$ 2,079.4
Allowance for depreciation and depletion	(1,043.8)		(1,028.4)
Property, plant and equipment, net	1,047.3	$ 1,051.0	1,051.0
Land Rights And Mineral Rights [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	549.6		549.6
Office And Information Technology [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	66.3		66.3
Buildings [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	85.5		86.8
Mining Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	594.0		594.4
Processing Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	619.8		617.0
Electric Power Facilities [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	57.0		57.0
Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	23.6		23.7
Asset Retirement Obligation [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	16.9		19.2
Other [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	30.3		30.3
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 48.1		$ 35.1