Contingencies - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended		6 Months Ended	9 Months Ended	12 Months Ended
	Apr. 30, 2016	Oct. 31, 2011	Sep. 30, 2017	Jun. 30, 2016	Sep. 30, 2016	Sep. 30, 2017	Dec. 31, 2015	Dec. 31, 2005	Dec. 31, 2016
Loss Contingencies [Line Items]
Other current liabilities			$ 14,493			$ 14,493			$ 13,083
Other liabilities			7,616			7,616			$ 9,235
New Almaden Mine [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			300
Other liabilities			1,200			1,200
Accrued balance						500
Originally 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			$ 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			$ 1,600
Accrued balance							$ 1,300
Total reserve			2,700			2,700
Other current liabilities			700			700
Other liabilities			$ 2,000			$ 2,000