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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
Sep. 30, 2019
Dec. 31, 2018
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross $ 2,851.7 $ 2,308.4
Allowance for depreciation and depletion (1,081.8) (1,022.4)
Property, plant and equipment, net 1,769.9 1,286.0
Land rights and mineral rights Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 549.7 549.6
Office and information technology Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 72.9 70.0
Buildings Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 101.2 87.2
Mining equipment Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 577.0 548.5
Processing equipment Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 755.5 645.8
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 29.2 25.2
Construction-in-progress Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross $ 668.9 $ 284.8