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PROPERTY, PLANT AND EQUIPMENT | 12 Months Ended
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Dec. 31, 2017
Property, Plant and Equipment [Abstract] |
PROPERTY, PLANT AND EQUIPMENT | NOTE 4 - PROPERTY, PLANT AND EQUIPMENT 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 |
We recorded depreciation expense of $78.8 million, $106.8 million and $119.2 million in the Statements of Consolidated Operations for the years ended December 31, 2017, 2016 and 2015, respectively. Our asset groups consist of the assets and liabilities of our mines and associated reserves. The lowest level of identifiable cash flows largely is at the U.S. Iron Ore and Asia Pacific Iron Ore segment levels. For the years ended December 31, 2017 and 2015, although certain factors indicated that the carrying value of certain asset groups may not be recoverable, an assessment for the potential impairment was performed and an impairment adjustment was not required. For the year ended December 31, 2016, there were no factors present that indicated the carrying value of certain asset groups would not be recoverable; therefore, additional impairment assessments were not required. 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 |
Accumulated depletion relating to mineral rights, which was recorded using the unit-of-production method, is included in Cost of goods sold and operating expenses. We recorded depletion expense of $6.8 million, $3.8 million and $7.4 million in the Statements of Consolidated Operations for the years ended December 31, 2017, 2016 and 2015, respectively.

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- Definition The entire disclosure for long-lived, physical assets used in the normal conduct of business and not intended for resale. Includes, but is not limited to, accounting policies and methodology, roll forwards, depreciation, depletion and amortization expense, including composite depreciation, accumulated depreciation, depletion and amortization expense, useful lives and method used, income statement disclosures, assets held for sale and public utility disclosures. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -URI http://asc.fasb.org/topic&trid=2155823 + Details
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