Business Segments - Revenue Components of Mineral Resources Segment (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Revenue from External Customer [Line Items]
Commercial/industrial revenues	$ 6,849	$ 7,000	$ 19,802	$ 23,633
Income (loss)	(2,950)	(1,243)	(2,598)	70
Operating Segments \| Mineral resources
Revenue from External Customer [Line Items]
Commercial/industrial revenues	3,187	2,652	11,927	12,852
Income (loss)	1,387	1,929	5,434	6,435
Operating Segments \| Mineral resources \| Oil and gas
Revenue from External Customer [Line Items]
Commercial/industrial revenues	383	917	769	1,693
Operating Segments \| Mineral resources \| Water sales
Revenue from External Customer [Line Items]
Commercial/industrial revenues	1,810	1,172	9,601	10,165
Operating Segments \| Mineral resources \| Rock aggregate
Revenue from External Customer [Line Items]
Commercial/industrial revenues	305	245	507	347
Operating Segments \| Mineral resources \| Cement
Revenue from External Customer [Line Items]
Commercial/industrial revenues	369	301	629	543
Operating Segments \| Mineral resources \| Land lease for oil exploration
Revenue from External Customer [Line Items]
Commercial/industrial revenues	25	17	126	104
Operating Segments \| Mineral resources \| Reimbursable costs
Revenue from External Customer [Line Items]
Commercial/industrial revenues	$ 295	$ 0	$ 295	$ 0

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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