Reporting Segments and Related Information - Revenue Components of Mineral Resources Segment (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Revenue from External Customer [Line Items]
Commercial/industrial revenues	$ 13,469	$ 5,702
Income (loss)	1,814	(3,473)
Mineral resources
Revenue from External Customer [Line Items]
Commercial/industrial revenues	9,131	2,001
Income (loss)	4,900	677
Mineral resources \| Oil and gas
Revenue from External Customer [Line Items]
Commercial/industrial revenues	538	376
Mineral resources \| Exploration leases
Revenue from External Customer [Line Items]
Commercial/industrial revenues	351	278
Mineral resources \| Rock aggregate
Revenue from External Customer [Line Items]
Commercial/industrial revenues	202	180
Mineral resources \| Water Sales
Revenue from External Customer [Line Items]
Commercial/industrial revenues	26	25
Mineral resources \| Cement
Revenue from External Customer [Line Items]
Commercial/industrial revenues	7,992	1,108
Mineral resources \| Reimbursables and other
Revenue from External Customer [Line Items]
Commercial/industrial revenues	$ 22	$ 34

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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