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Annual report pursuant to Section 13 and 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
Dec. 31, 2019
Dec. 31, 2018
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross $ 3,027.4 $ 2,308.4
Allowance for depreciation and depletion (1,098.4) (1,022.4)
Property, plant and equipment, net 1,929.0 1,286.0
Land Rights And Mineral Rights [Member] Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 549.7 549.6
Office And Information Technology [Member] Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 71.9 70.0
Buildings [Member] Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 157.8 87.2
Mining Equipment [Member] Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 581.9 548.5
Processing Equipment [Member] Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 791.8 645.8
Electric Power Facilities [Member] Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 81.9 58.7
Land Improvements [Member] Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 32.5 23.8
Asset Retirement Obligation [Member] Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 1.7 14.8
Other Assets [Member] Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 27.9 25.2
Construction in Progress [Member] Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross $ 730.3 $ 284.8