Contingencies - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended	12 Months Ended					99 Months Ended
	Jan. 31, 2019	Apr. 30, 2016	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2005	Dec. 31, 2019
Loss Contingencies [Line Items]
Other current liabilities			$ 16,701	$ 18,324	$ 16,701				$ 18,324
Other liabilities			19,794	22,582	19,794				22,582
Lease guarantee contingency			10,402	10,724	10,402				10,724
New Idria Mercury Mine [Member]
Loss Contingencies [Line Items]
Financial assurance required to be provided to EPA to secure performance			2,000
Prepayment amount	$ 200
New Almaden Mine (Formerly Referred to as Guadalupe River Watershed) [Member] | Natural Resource Damage Claim [Member]
Loss Contingencies [Line Items]
Total reserve			1,500	1,500	1,500				1,500
Other current liabilities				300					300
Other liabilities				1,200					1,200
Expense recognized							$ 1,200	$ 800
Accrued balance				$ 500
Original estimated project costs		$ 1,600
Revised estimated project costs, Low Estimate		3,300
Revised estimated project costs, High Estimate		$ 4,400
Lawn and Garden Indemnification Claim [Member] | Guarantee Obligation [Member]
Loss Contingencies [Line Items]
Lease expiring period				September 2025
Annual rent				$ 2,000
Liabilities and related pre tax charges					10,300
Lease guarantee contingency			10,400	10,700	10,400				10,700
Pending Litigation [Member] | New Idria Mercury Mine [Member] | EPA Notice Letter [Member]
Loss Contingencies [Line Items]
Loss contingencies, payments									3,200
Loss contingency, Loss in period				4,000	200	$ 1,300			9,900
Total reserve			$ 3,400	6,700	$ 3,400				6,700
Other current liabilities				1,200					1,200
Other liabilities				$ 5,500					$ 5,500