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Fair Value Measurement - Realized Unrealized Derivatives (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivatives, Fair Value [Line Items]
Realized gain (loss)		$ 228	$ 1,000	$ 227
Unrealized gain (loss)	$ (2,303)	(252)	(2,732)	284
Gold
Derivatives, Fair Value [Line Items]
Realized gain (loss)		177	14	129
Unrealized gain (loss)	(38)	(138)	62	32
Silver
Derivatives, Fair Value [Line Items]
Realized gain (loss)		236	(53)	190
Unrealized gain (loss)	13	(236)	162	(9)
Copper
Derivatives, Fair Value [Line Items]
Realized gain (loss)		23	53	37
Unrealized gain (loss)	(103)	(7)	(203)	19
Lead
Derivatives, Fair Value [Line Items]
Realized gain (loss)		12	(5)	48
Unrealized gain (loss)	(138)	2	(161)	60
Zinc
Derivatives, Fair Value [Line Items]
Realized gain (loss)		(220)	991	(177)
Unrealized gain (loss)	$ (2,037)	$ 127	$ (2,592)	$ 182

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- Definition

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+ References

No definition available.

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- Definition

The difference between the book value and the sale price of options, swaps, futures, forward contracts, and other derivative instruments. This element refers to the gain (loss) included in earnings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.13(h))
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

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- Definition

The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

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