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Quarterly report pursuant to Section 13 or 15(d)

PROPERTY, PLANT AND EQUIPMENT (Value Of Each Of The Major Classes Of Consolidated Depreciable Assets) (Details)

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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] Ìý Ìý
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 Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 549.6 549.6
Office and information technology Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 70.5 70.0
Buildings Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 87.4 87.2
Mining equipment Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 568.4 548.5
Processing equipment Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 660.9 645.8
Electric power facilities Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 58.7 58.7
Land improvements Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 23.8 23.8
Asset retirement obligation Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 14.8 14.8
Other Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 25.5 25.2
Construction-in-progress Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross $ 392.0 $ 284.8