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Reporting Segments and Related Information - Revenue Components of Mineral Resources Segment (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Revenue from External Customer [Line Items] |  |  |
Segment revenues | $ 35,657 | $ 45,577 | $ 51,147
Operating Segments |  |  |
Revenue from External Customer [Line Items] |  |  |
Segment revenues | 35,657 | 45,577 | 51,147
Operating Segments | Mineral resources |  |  |
Revenue from External Customer [Line Items] |  |  |
Segment revenues | 5,983 | 14,153 | 15,116
Operating Segments | Mineral resources | Oil and gas |  |  |
Revenue from External Customer [Line Items] |  |  |
Segment revenues | 1,659 | 1,549 | 2,661
Operating Segments | Mineral resources | Rock aggregate |  |  |
Revenue from External Customer [Line Items] |  |  |
Segment revenues | 1,072 | 1,164 | 870
Operating Segments | Mineral resources | Cement |  |  |
Revenue from External Customer [Line Items] |  |  |
Segment revenues | 1,614 | 1,299 | 1,263
Operating Segments | Mineral resources | Exploration leases |  |  |
Revenue from External Customer [Line Items] |  |  |
Segment revenues | 102 | 176 | 157
Operating Segments | Mineral resources | Water sales |  |  |
Revenue from External Customer [Line Items] |  |  |
Segment revenues | 1,254 | 9,601 | 10,165
Operating Segments | Mineral resources | Reimbursable |  |  |
Revenue from External Customer [Line Items] |  |  |
Segment revenues | $ 282 | $ 364 | $ 0

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- Definition Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.1) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 + Details
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