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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, 2016 | Dec. 31, 2015
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Property, Plant and Equipment [Line Items] |  |
Property, plant and equipment, gross | $ 1,947.0 | $ 1,970.3
Allowance for depreciation and depletion | (937.4) | (911.3)
Property, plant and equipment, net | 1,009.6 | 1,059.0
Land Rights And Mineral Rights [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Property, plant and equipment, gross | 500.5 | 500.5
Office And Information Technology [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Property, plant and equipment, gross | 57.9 | 71.0
Buildings [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Property, plant and equipment, gross | 60.5 | 60.4
Mining Equipment [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Property, plant and equipment, gross | 597.4 | 594.0
Processing Equipment [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Property, plant and equipment, gross | 519.4 | 516.8
Electric Power Facilities [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Property, plant and equipment, gross | 47.3 | 46.4
Land Improvements [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Property, plant and equipment, gross | 24.8 | 24.8
Asset Retirement Obligation [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Property, plant and equipment, gross | 67.1 | 87.9
Other [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Property, plant and equipment, gross | 29.2 | 28.2
Construction in Progress [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Property, plant and equipment, gross | $ 42.9 | $ 40.3

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- Definition Amount before accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.13) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Amount after accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.13) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 13 -Subparagraph a -Article 5 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 8 -Article 7 + Details
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