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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, 2018
Jan. 01, 2018
Dec. 31, 2017
Property, Plant and Equipment [Line Items] Ìý Ìý Ìý
Property, plant and equipment, gross $ 2,308.4 Ìý $ 2,000.7
Accumulated Depreciation and Depletion, Property, Plant and Equipment (1,022.4) Ìý (966.9)
Property, plant and equipment, net 1,286.0 $ 1,033.8 1,033.8
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 70.0 Ìý 65.8
Buildings [Member] Ìý Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý Ìý
Property, plant and equipment, gross 87.2 Ìý 85.2
Mining Equipment [Member] Ìý Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý Ìý
Property, plant and equipment, gross 548.5 Ìý 533.9
Processing Equipment [Member] Ìý Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý Ìý
Property, plant and equipment, gross 645.8 Ìý 610.9
Electric Power Facilities [Member] Ìý Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý Ìý
Property, plant and equipment, gross 58.7 Ìý 56.9
Land Improvements [Member] Ìý Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý Ìý
Property, plant and equipment, gross 23.8 Ìý 23.7
Asset Retirement Obligation [Member] Ìý Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý Ìý
Property, plant and equipment, gross 14.8 Ìý 16.9
Other Assets [Member] 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 [Member] Ìý Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý Ìý
Property, plant and equipment, gross $ 284.8 Ìý $ 32.6