Investment in Affiliate - Combined Balance Sheets (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Current Assets
Cash and cash equivalents	$ 12,398			$ 150,146
Other current assets	1,058			3,879
Total current assets	14,736			155,752
Non-Current Assets
Total Assets	408,385			265,410	$ 112,933
Non-Current Liabilities
Dowa Term Loan	12,583
Owners' Capital
Paid-in capital	542,193			409,728
Accumulated deficit	(150,551)			(147,423)
Total owners' capital	391,759	$ 280,777	$ 262,141	261,386	$ 121,524	$ 121,604	$ 132,662	$ 149,391
Total Liabilities and Owners' Capital	408,385			265,410
Los Gatos Joint Venture ("LGJV")
Current Assets
Cash and cash equivalents	9,941			1,676
Receivables	6,961			3,988
Inventories	10,027			10,315
VAT receivable	47,096			50,732
Other current assets	2,543			2,891
Total current assets	76,568			69,602
Non-Current Assets
Mine development, net	222,128			202,874
Property, plant and equipment, net	192,675			196,942
Total noncurrent assets	414,803			399,816
Total Assets	491,371			469,418
Current Liabilities
Accounts payable and accrued liabilities	32,951			35,767
Related party payable	1,299			1,703
Accrued interest	45			101
Unearned revenue				3,276
Equipment loans	6,365			7,084
Dowa Term Loan				31,826
Working Capital Facility				60,000
Total current liabilities	40,660			139,757
Non-Current Liabilities
Dowa Term Loan				187,767
Equipment loans	1,224			6,120
Reclamation obligations	12,846			12,162
Total noncurrent liabilities	14,070			206,049
Owners' Capital
Capital contributions	540,638			271,368
Paid-in capital	18,405			16,366
Accumulated deficit	(122,402)			(164,122)
Total owners' capital	436,641			123,612
Total Liabilities and Owners' Capital	$ 491,371			$ 469,418