Contingencies - Additional Information (Details) - USD ($)	1 Months Ended		3 Months Ended				9 Months Ended		12 Months Ended		108 Months Ended
	Jan. 31, 2019	Apr. 30, 2016	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2016	Dec. 31, 2005	Sep. 30, 2020	Dec. 31, 2019
Loss Contingencies [Line Items]
Loss contingency, Loss in period								$ 4,000,000.0
Other current liabilities			$ 16,454,000				$ 16,454,000				$ 16,454,000	$ 18,324,000
Other liabilities			12,933,000				12,933,000				12,933,000	$ 22,582,000
New Idria Mercury Mine [Member]
Loss Contingencies [Line Items]
Financial assurance required to be provided to EPA to secure performance						$ 2,000,000
Prepayment amount	$ 200,000
Loss contingency, Loss in period				$ 0
New Almaden Mine (Formerly Referred to as Guadalupe River Watershed) [Member] | Natural Resource Damage Claim [Member]
Loss Contingencies [Line Items]
Total reserve			1,500,000				1,500,000				1,500,000
Other current liabilities			300,000				300,000				300,000
Other liabilities			1,200,000				1,200,000				1,200,000
Expense recognized			0	$ 0			0	0	$ 1,200,000	$ 800,000
Accrued balance							500,000
Original estimated project costs		$ 1,600,000
Revised estimated project costs, Low Estimate		3,300,000
Revised estimated project costs, High Estimate		$ 4,400,000
Pending Litigation [Member] | New Idria Mercury Mine [Member] | EPA Notice Letter [Member]
Loss Contingencies [Line Items]
Loss contingencies, payments											3,200,000
Loss contingency, Loss in period			0		$ 4,000,000.0		0	$ 4,000,000.0			9,900,000
Total reserve			6,700,000				6,700,000				6,700,000
Other current liabilities			1,100,000				1,100,000				1,100,000
Other liabilities			$ 5,600,000				$ 5,600,000				$ 5,600,000