Deferred Gain on Sale of Royalty (Details) - USD ($) $ in Thousands		3 Months Ended	9 Months Ended
	May 29, 2020	Sep. 30, 2021	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Other Liabilities Disclosure [Abstract]
Proceeds from royalty obligation	$ 30,000		$ 0	$ 30,000
Smelter royalty obligation, percentage	1.50%
Smelter royalty obligation, right to repurchase percentage	33.30%
Smelter royalty obligation, right to repurchase percentage, net of returns	0.50%
Amortization of royalty obligation		$ 100	100
Payments for royalty obligations		1,000	2,000
Deferred gain on sale of royalty		$ 125	$ 125		$ 124