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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, 2018
Jan. 01, 2018
Dec. 31, 2017
Property, Plant and Equipment [Line Items] Ìý Ìý Ìý
Property, plant and equipment, gross $ 2,152.9 Ìý $ 2,000.7
Allowance for depreciation and depletion (1,008.1) Ìý (966.9)
Property, plant and equipment, net 1,144.8 $ 1,033.8 1,033.8
Land rights and mineral rights Ìý Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý Ìý
Property, plant and equipment, gross 549.6 Ìý 549.6
Office and information technology Ìý Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý Ìý
Property, plant and equipment, gross 67.8 Ìý 65.8
Buildings Ìý Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý Ìý
Property, plant and equipment, gross 84.1 Ìý 85.2
Mining equipment Ìý Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý Ìý
Property, plant and equipment, gross 538.7 Ìý 533.9
Processing equipment Ìý Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý Ìý
Property, plant and equipment, gross 619.0 Ìý 610.9
Electric power facilities Ìý Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý Ìý
Property, plant and equipment, gross 58.7 Ìý 56.9
Land improvements Ìý Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý Ìý
Property, plant and equipment, gross 24.2 Ìý 23.7
Asset retirement obligation Ìý Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý Ìý
Property, plant and equipment, gross 16.9 Ìý 16.9
Other Ìý Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý Ìý
Property, plant and equipment, gross 25.2 Ìý 25.2
Construction in-progress Ìý Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý Ìý
Property, plant and equipment, gross $ 168.7 Ìý $ 32.6