PROPERTY, PLANT AND EQUIPMENT (Value Of Each Of The Major Classes Of Consolidated Depreciable Assets) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 2,071.3		$ 2,000.7
Allowance for depreciation and depletion	(990.0)		(966.9)
Property, plant and equipment, net	1,081.3	$ 1,033.8	1,033.8
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	65.1		65.8
Buildings
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	84.0		85.2
Mining equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	531.2		533.9
Processing equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	616.0		610.9
Electric power facilities
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	56.9		56.9
Land improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	24.2		23.7
Asset retirement obligation
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	16.9		16.9
Other
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	25.2		25.2
Construction in-progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 102.2		$ 32.6