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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, 2018
Jan. 01, 2018
Dec. 31, 2017
Property, Plant and Equipment [Line Items] Ìý Ìý Ìý
Property, plant and equipment, gross $ 2,091.1 Ìý $ 2,079.4
Allowance for depreciation and depletion (1,043.8) Ìý (1,028.4)
Property, plant and equipment, net 1,047.3 $ 1,051.0 1,051.0
Land Rights And Mineral Rights [Member] 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 [Member] Ìý Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý Ìý
Property, plant and equipment, gross 66.3 Ìý 66.3
Buildings [Member] Ìý Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý Ìý
Property, plant and equipment, gross 85.5 Ìý 86.8
Mining Equipment [Member] Ìý Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý Ìý
Property, plant and equipment, gross 594.0 Ìý 594.4
Processing Equipment [Member] Ìý Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý Ìý
Property, plant and equipment, gross 619.8 Ìý 617.0
Electric Power Facilities [Member] Ìý Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý Ìý
Property, plant and equipment, gross 57.0 Ìý 57.0
Land Improvements [Member] Ìý Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý Ìý
Property, plant and equipment, gross 23.6 Ìý 23.7
Asset Retirement Obligation [Member] Ìý Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý Ìý
Property, plant and equipment, gross 16.9 Ìý 19.2
Other [Member] Ìý Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý Ìý
Property, plant and equipment, gross 30.3 Ìý 30.3
Construction in Progress [Member] Ìý Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý Ìý
Property, plant and equipment, gross $ 48.1 Ìý $ 35.1