Contingencies - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended				6 Months Ended	12 Months Ended
	Apr. 30, 2016	Oct. 31, 2011	Sep. 30, 2017	Jun. 30, 2016	Jun. 30, 2015	Sep. 30, 2014	Sep. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2005
Loss Contingencies [Line Items]
Other current liabilities								$ 15,472	$ 12,911
Other liabilities								8,236	9,203
New Idria Mercury Mine [Member]
Loss Contingencies [Line Items]
Expected future EPA oversite costs recognized								1,000
Financial assurance required to be provided to EPA								$ 2,000
Financial assurance provision expected period following execution of AOC								30 days
New Almaden Mine (Formerly Referred to as Guadalupe River Watershed) [Member] | Natural Resource Damage Claim [Member]
Loss Contingencies [Line Items]
Expense recognized							$ 1,200				$ 800
Total reserve								$ 1,500	1,500
Other current liabilities								300
Other liabilities								1,200
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]
Loss Contingencies [Line Items]
Indemnification claims								10,000
Escrow deposit								$ 8,600
Escrow deposit due to be settled date								2016-08
Pending Litigation [Member] | New Idria Mercury Mine [Member] | EPA Notice Letter [Member]
Loss Contingencies [Line Items]
Expense recognized		$ 1,900		$ 1,000
Additional reserve			$ 300
Loss contingency costs								$ 1,600
Accrued balance										$ 1,300
Total reserve								3,600	2,500
Other current liabilities								1,000
Other liabilities								2,600
Settled Litigation [Member] | Patent Infringement Against Orbis Corp [Member] | Loss From Patent Infringement [Member]
Loss Contingencies [Line Items]
Accrued balance						$ 3,000
Litigation settlement, amount									$ 200
Contingency accrual reversal					$ 3,000
Settled Litigation [Member] | Patent Infringement Against Orbis Corp [Member] | Loss From Patent Infringement [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Litigation settlement, amount								$ 3,100