Contingencies - Additional Information (Details) - USD ($)	1 Months Ended		3 Months Ended					9 Months Ended		12 Months Ended		96 Months Ended
	Jan. 31, 2019	Apr. 30, 2016	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2016	Dec. 31, 2005	Sep. 30, 2019
Loss Contingencies [Line Items]
Other current liabilities			$ 18,035,000	$ 16,701,000				$ 18,035,000				$ 18,035,000
Other liabilities			22,706,000	19,794,000				22,706,000				22,706,000
Lease guarantee contingency			10,642,000	10,402,000				10,642,000				10,642,000
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
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
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
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
Liabilities and related pre tax charges					10,300,000
Lease guarantee contingency			10,600,000	$ 10,400,000				10,600,000				10,600,000
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		$ 0			4,000,000	$ 0			9,900,000
Total reserve			6,700,000					6,700,000				6,700,000
Other current liabilities			1,200,000					1,200,000				1,200,000
Other liabilities			$ 5,500,000					$ 5,500,000				$ 5,500,000