Contingencies - Additional Information (Details) - USD ($)	1 Months Ended	3 Months Ended			6 Months Ended		12 Months Ended			81 Months Ended
	Apr. 30, 2016	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2005	Jun. 30, 2018
Loss Contingencies [Line Items]
Other current liabilities		$ 15,687,000			$ 15,687,000		$ 15,472,000			$ 15,687,000
Other liabilities		8,970,000			8,970,000		8,236,000			8,970,000
Discontinued operations pre-tax charge		0		$ 332,000	1,225,000	$ 949,000
New Idria Mercury Mine [Member]
Loss Contingencies [Line Items]
Expected future EPA oversite costs recognized							$ 1,000,000
Financial assurance required to be provided to EPA to secure performance					$ 2,000,000
Financial assurance provision expected period following execution of AOC					30 days
Loss Contingency, Loss in Period		0			$ 0
New Almaden Mine [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
Originally estimated project costs	$ 1,600,000
Revised estimated project costs, Low Estimate	3,300,000
Revised estimated project costs, High Estimate	$ 4,400,000
Lawn and Garden Indemnification Claim [Member]
Loss Contingencies [Line Items]
Indemnification claims					10,000,000
Escrow deposit		8,600,000			$ 8,600,000					8,600,000
Escrow deposit due to be settled date					2016-08
Discontinued operations pre-tax charge			$ 1,225,000
Amount of escrow deposits expected to release upon settlement of agreement		7,400,000
Pending Litigation [Member] | New Idria Mercury Mine [Member] | EPA Notice Letter [Member]
Loss Contingencies [Line Items]
Loss contingencies, payments										2,300,000
Loss Contingency, Loss in Period				$ 0		$ 0				5,700,000
Total reserve		3,400,000			$ 3,400,000					3,400,000
Other current liabilities		700,000			700,000					700,000
Other liabilities		$ 2,700,000			$ 2,700,000					$ 2,700,000