PROPERTY, PLANT AND EQUIPMENT (Value Of Each Of The Major Classes Of Consolidated Depreciable Assets) (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 2,658.0	$ 2,308.4
Allowance for depreciation and depletion	(1,060.7)	(1,022.4)
Property, plant and equipment, net	1,597.3	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	70.6	70.0
Buildings
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	95.4	87.2
Mining equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	575.8	548.5
Processing equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	740.3	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	14.9	14.8
Other
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	27.6	25.2
Construction-in-progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 501.3	$ 284.8