Asset Impairment Review (Narrative) (Details) - USD ($) $ in Thousands	1 Months Ended	9 Months Ended
Oct. 31, 2017	Sep. 30, 2017
Subsequent Event [Member]
Impairment Effects on Earnings Per Share [Line Items]
Proceeds from sale of land	$ 4,900
Carlisle Assets [Member]
Impairment Effects on Earnings Per Share [Line Items]
Mine carrying value		$ 112,000
Asset impairment charge		0
Bulldog Mine [Member]
Impairment Effects on Earnings Per Share [Line Items]
Aggregate net carrying value of land		$ 15,000
Bulldog Mine [Member] \| Subsequent Event [Member]
Impairment Effects on Earnings Per Share [Line Items]
Asset impairment charge	$ 0
Land buy-back option period	8 years

Land buy-back option period

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- Definition

Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 45
-Paragraph 4
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- Definition

Amount classified as property, plant and equipment attributable to disposal group held for sale or disposed of.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 5B
-Subparagraph (e)
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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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- Definition

Mineral properties, net of adjustments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 360
-Section 25
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-URI http://asc.fasb.org/extlink&oid=56951519&loc=d3e64895-109465

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- Definition

The cash inflow from the sale of land held for use; excludes sales of land held as inventory or investments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Name Accounting Standards Codification
-Glossary Investing Activities
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