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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, 2017
Dec. 31, 2016
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross $ 2,006.3 $ 1,941.3
Allowance for depreciation and depletion (1,012.5) (956.9)
Property, plant and equipment, net 993.8 984.4
Land Rights And Mineral Rights [Member] Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 500.7 500.5
Office And Information Technology [Member] Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 66.2 65.1
Buildings [Member] Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 80.0 67.9
Mining Equipment [Member] Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 585.4 592.2
Processing Equipment [Member] Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 607.9 552.0
Electric Power Facilities [Member] Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 57.0 49.4
Land Improvements [Member] Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 23.7 23.5
Asset Retirement Obligation [Member] Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 19.6 19.8
Other [Member] Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 30.4 28.1
Construction in Progress [Member] Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross $ 35.4 $ 42.8