Reporting Segments and Related Information - Revenue Components of Mineral Resources Segment (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Revenue from External Customer [Line Items]
Commercial/industrial revenues	$ 11,908	$ 13,079	$ 23,975	$ 32,881
Income (loss)	(1,414)	(1,765)	(6,720)	(4,363)
Mineral resources
Revenue from External Customer [Line Items]
Commercial/industrial revenues	1,142	1,125	4,662	13,052
Income (loss)	614	458	2,281	5,892
Mineral resources \| Oil and gas
Revenue from External Customer [Line Items]
Commercial/industrial revenues	356	394	1,161	1,163
Mineral resources \| Water sales
Revenue from External Customer [Line Items]
Commercial/industrial revenues	0	0	1,254	9,601
Mineral resources \| Rock aggregate
Revenue from External Customer [Line Items]
Commercial/industrial revenues	262	306	690	813
Mineral resources \| Cement
Revenue from External Customer [Line Items]
Commercial/industrial revenues	398	367	1,195	996
Mineral resources \| Land lease for oil exploration
Revenue from External Customer [Line Items]
Commercial/industrial revenues	51	25	76	151
Mineral resources \| Reimbursable costs
Revenue from External Customer [Line Items]
Commercial/industrial revenues	$ 75	$ 33	$ 286	$ 328

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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- Definition

Amount of income (loss) from continuing operations before deduction of income tax expense (benefit) and income (loss) attributable to noncontrolling interest, and addition of income (loss) from equity method investments.

+ 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=63488584&loc=d3e20235-122688

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