Contingencies - Additional Information (Details) - USD ($)	1 Months Ended	3 Months Ended				9 Months Ended		12 Months Ended		84 Months Ended
	Apr. 30, 2016	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2016	Dec. 31, 2005	Sep. 30, 2018	Dec. 31, 2017
Loss Contingencies [Line Items]
Other current liabilities		$ 16,814,000				$ 16,814,000				$ 16,814,000	$ 15,472,000
Other liabilities		19,264,000				19,264,000				19,264,000	$ 8,236,000
Discontinued operations pre-tax charge		123,000			$ (343,000)	1,348,000	$ 606,000
New Idria Mercury Mine [Member]
Loss Contingencies [Line Items]
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
Lawn and Garden Indemnification Claim [Member] | Guarantee Obligation [Member]
Loss Contingencies [Line Items]
Lease expiring period						September 2025
Annual rent						$ 2,000,000
Lawn and Garden Indemnification Claim [Member] | Guarantee Obligation [Member] | Other Expenses [member]
Loss Contingencies [Line Items]
Pre tax charges		10,300,000
Lawn and Garden Indemnification Claim [Member] | Guarantee Obligation [Member] | Other Liabilities [Member]
Loss Contingencies [Line Items]
Lease guarantee liability		10,300,000				10,300,000				10,300,000
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					$ 300,000		$ 300,000			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