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PROPERTY, PLANT AND EQUIPMENT (Tables) | 12 Months Ended
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Dec. 31, 2017
Property, Plant and Equipment [Abstract] |
Value Of Each Of The Major Classes Of Consolidated Depreciable Assets | The following table indicates the carrying value of each of the major classes of our consolidated depreciable assets as of December 31, 2017 and 2016:
| (In Millions)
| December 31,
| 2017 |  | 2016
Land rights and mineral rights | $ | 549.6 |  |  | $ | 500.5 |
Office and information technology | 66.3 |  |  | 65.1 |
Buildings | 86.8 |  |  | 67.9 |
Mining equipment | 594.4 |  |  | 592.2 |
Processing equipment | 617.0 |  |  | 552.0 |
Electric power facilities | 57.0 |  |  | 49.4 |
Land improvements | 23.7 |  |  | 23.5 |
Asset retirement obligation | 19.2 |  |  | 19.8 |
Other | 30.3 |  |  | 28.1 |
Construction in-progress | 35.1 |  |  | 42.8 |
| 2,079.4 |  |  | 1,941.3 |
Allowance for depreciation and depletion | (1,028.4 | ) |  | (956.9 | )
| $ | 1,051.0 |  |  | $ | 984.4 |
Book Value of Land and Mineral Rights Disclosure [Table Text Block] | The net book value of the land rights and mineral rights as of December 31, 2017 and 2016 is as follows:
| (In Millions)
| December 31,
| 2017 |  | 2016
Land rights | $ | 12.4 |  |  | $ | 11.6 |
Mineral rights: |  |  |
Cost | $ | 537.2 |  |  | $ | 488.9 |
Depletion | (119.1 | ) |  | (112.2 | )
Net mineral rights | $ | 418.1 |  |  | $ | 376.7 |

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- Definition Tabular disclosure of physical assets used in the normal conduct of business and not intended for resale. Includes, but is not limited to, balances by class of assets, depreciation and depletion expense and method used, including composite depreciation, and accumulated deprecation. + References Reference 1: 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 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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