Business Segments - Revenue Components of Mineral Resources Segment (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2016	Mar. 31, 2015
Revenue from External Customer [Line Items]
Segment revenues	$ 12,953	$ 16,633
Income (loss)	352	1,313
Mineral resources
Revenue from External Customer [Line Items]
Segment revenues	8,740	10,200
Operating Segments \| Mineral resources
Revenue from External Customer [Line Items]
Segment revenues	8,740	10,200
Income (loss)	4,047	4,426
Operating Segments \| Mineral resources \| Oil and gas
Revenue from External Customer [Line Items]
Segment revenues	386	776
Operating Segments \| Mineral resources \| Water sales
Revenue from External Customer [Line Items]
Segment revenues	7,791	8,993
Operating Segments \| Mineral resources \| Rock aggregate
Revenue from External Customer [Line Items]
Segment revenues	202	102
Operating Segments \| Mineral resources \| Cement
Revenue from External Customer [Line Items]
Segment revenues	260	242
Operating Segments \| Mineral resources \| Land lease for oil exploration
Revenue from External Customer [Line Items]
Segment revenues	$ 101	$ 87

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Sum of operating profit and nonoperating income or expense before Income or Loss from equity method investments, income taxes, extraordinary items, and noncontrolling interest.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(h)(1)(i))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

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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=26872669&loc=d3e20235-122688

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