Amount of deferred revenue as of balance sheet date. Deferred revenue represents collections of cash or other assets related to a revenue producing activity for which revenue has not yet been recognized. Generally, an entity records deferred revenue when it receives consideration from a customer before achieving certain criteria that must be met for revenue to be recognized in conformity with GAAP.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 605
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 13.A.3)
-URI http://asc.fasb.org/extlink&oid=65893310&loc=d3e214044-122780

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 605
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 13.A.4(a).Q1)
-URI http://asc.fasb.org/extlink&oid=65893310&loc=d3e214044-122780

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- Definition

The cash inflow associated with the deconsolidation of a previously consolidated subsidiary or sale of an entity that is related to it but not strictly controlled.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Investing Activities
-URI http://asc.fasb.org/extlink&oid=6516133

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3179-108585

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- Definition

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.

+ References

No definition available.

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The gains (losses) included in results of operations resulting from the sale or disposal of mineral property, net of transaction costs, which do not qualify for treatment as discontinued operations. This item does not include any gain (loss) recognized on the sale of oil and gas property or timber property.

+ References

No definition available.

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- Definition

Represents the percentage of net smelter return royalty on any future productions from specified claims.

+ References

No definition available.

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Mineral Properties, Long Valley (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Gain on disposal of mineral property, net	$ 358	$ 150	$ 1,958
Long Valley [Member] \| Sold
Property, Plant and Equipment [Line Items]
Proceeds from sale of gold project	358
Gain on disposal of mineral property, net	$ 358
Long Valley [Member] \| Sold \| Minimum [Member]
Property, Plant and Equipment [Line Items]
Net smelter return royalty (as a percent)	0.50%
Long Valley [Member] \| Sold \| Maximum [Member]
Property, Plant and Equipment [Line Items]
Net smelter return royalty (as a percent)	2.00%
Long Valley [Member] \| Sold \| One Month after the Start of Production [Member]
Property, Plant and Equipment [Line Items]
Future payment to be received	$ 500
Long Valley [Member] \| Sold \| On or Prior to the First Anniversary of the Start of Production [Member]
Property, Plant and Equipment [Line Items]
Future payment to be received	$ 500