Business Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Operating results for the business segments
Revenues	$ 71,842	$ 69,473	$ 217,544	$ 191,126
Income (loss) from operations	6,046	1,403	10,354	(6,968)
Write-down of assets held for sale to fair value	451		2,742
Exploration		753	244	2,556
Equity in the net loss of joint venture/equity method investee	861	159	2,082	159
GRE [Member]
Operating results for the business segments
Revenues	71,672	69,473	217,373	191,126
Income (loss) from operations	9,208	4,773	22,826	4,352
Write-down of assets held for sale to fair value
Exploration
Equity in the net loss of joint venture/equity method investee	767	159	1,988	159
Afek [Member]
Operating results for the business segments
Revenues
Income (loss) from operations	(54)	(1,011)	(541)	(3,611)
Write-down of assets held for sale to fair value
Exploration		753	244	2,256
Equity in the net loss of joint venture/equity method investee
GOGAS [Member]
Operating results for the business segments
Revenues	170		171
Income (loss) from operations	(1,062)	(158)	(5,307)	(350)
Write-down of assets held for sale to fair value	451		2,742
Exploration
Equity in the net loss of joint venture/equity method investee	94		94
Corporate [Member]
Operating results for the business segments
Revenues
Income (loss) from operations	(2,046)	(2,201)	(6,624)	(7,359)
Write-down of assets held for sale to fair value
Exploration
Equity in the net loss of joint venture/equity method investee

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Exploration expenses (including prospecting) related to oil and gas producing entities and would be included in operating expenses of that entity. Costs incurred in identifying areas that may warrant examination and in examining specific areas that are considered to have prospects of containing oil and gas reserves, including costs of drilling exploratory wells and exploratory-type stratigraphic test wells. Exploration costs may be incurred both before acquiring the related property (sometimes referred to in part as prospecting costs) and after acquiring the property. Principal types of exploration costs, which include depreciation and applicable operating costs of support equipment and facilities and other costs of exploration activities, are: (i) Costs of topographical, geographical and geophysical studies, rights of access to properties to conduct those studies, and salaries and other expenses of geologists, geophysical crews, and others conducting those studies. Collectively, these are sometimes referred to as geological and geophysical or "G&G" costs. (ii) Costs of carrying and retaining undeveloped properties, such as delay rentals, ad valorem taxes on properties, legal costs for title defense, and the maintenance of land and lease records. (iii) Dry hole contributions and bottom hole contributions. (iv) Costs of drilling and equipping exploratory wells. (v) Costs of drilling exploratory-type stratigraphic test wells.

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The aggregate amount of write-downs for impairments recognized during the period for long-lived assets held for abandonment, exchange or sale.

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