PROPERTY, PLANT AND EQUIPMENT (Value Of Each Of The Major Classes Of Consolidated Depreciable Assets) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 2,451.6	$ 2,308.4
Allowance for depreciation and depletion	(1,041.3)	(1,022.4)
Property, plant and equipment, net	1,410.3	1,286.0
Land rights and mineral rights
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	549.6	549.6
Office and information technology
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	70.5	70.0
Buildings
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	87.4	87.2
Mining equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	568.4	548.5
Processing equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	660.9	645.8
Electric power facilities
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	58.7	58.7
Land improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	23.8	23.8
Asset retirement obligation
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	14.8	14.8
Other
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	25.5	25.2
Construction-in-progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 392.0	$ 284.8