Asset Impairment Review (Narrative) (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended
Oct. 31, 2017	Mar. 31, 2018
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		$ 108,000
Asset impairment charge		0
Bulldog Mine [Member]
Impairment Effects on Earnings Per Share [Line Items]
Asset impairment charge		0
Land buy-back option period	8 years
Aggregate net carrying value of land		$ 15,000

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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

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-Publisher FASB
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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
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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
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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

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