PROPERTY, PLANT AND EQUIPMENT (Value Of Each Of The Major Classes Of Consolidated Depreciable Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 2,308.4		$ 2,000.7
Accumulated Depreciation and Depletion, Property, Plant and Equipment	(1,022.4)		(966.9)
Property, plant and equipment, net	1,286.0	$ 1,033.8	1,033.8
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	70.0		65.8
Buildings [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	87.2		85.2
Mining Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	548.5		533.9
Processing Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	645.8		610.9
Electric Power Facilities [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	58.7		56.9
Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	23.8		23.7
Asset Retirement Obligation [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	14.8		16.9
Other Assets [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	25.2		25.2
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 284.8		$ 32.6