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Fair Value Measurement - Realized Unrealized Derivatives, net (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Derivatives, Fair Value [Line Items]
Realized (loss) gain	$ (246)	$ 653	$ 204	$ 250
Unrealized (loss) gain	(232)	(103)	(48)	(232)
Total realized/ unrealized derivatives, net	(478)	550	156	18
Gold
Derivatives, Fair Value [Line Items]
Realized (loss) gain	(110)	132	(133)	651
Unrealized (loss) gain	28	(11)	58	(209)
Total realized/ unrealized derivatives, net	(82)	121	(75)	442
Silver
Derivatives, Fair Value [Line Items]
Realized (loss) gain	(295)	339	(123)	451
Unrealized (loss) gain	(11)	(47)	10	(290)
Total realized/ unrealized derivatives, net	(306)	292	(113)	161
Copper
Derivatives, Fair Value [Line Items]
Realized (loss) gain	(4)	89	63	20
Unrealized (loss) gain	(16)	(24)	(24)	(9)
Total realized/ unrealized derivatives, net	(20)	65	39	11
Lead
Derivatives, Fair Value [Line Items]
Realized (loss) gain	72	16	146	(143)
Unrealized (loss) gain	(84)	(10)	(63)	41
Total realized/ unrealized derivatives, net	(12)	6	83	(102)
Zinc
Derivatives, Fair Value [Line Items]
Realized (loss) gain	91	77	251	(729)
Unrealized (loss) gain	(149)	(11)	(29)	235
Total realized/ unrealized derivatives, net	$ (58)	$ 66	$ 222	$ (494)

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- Definition

Net Increase or Decrease in the fair value of the embedded derivative or group of embedded derivatives included in earnings in the period.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 15
-URI http://asc.fasb.org/subtopic&trid=2229187

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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

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
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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.

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