Contingencies - Additional Information (Details) - USD ($)	1 Months Ended		3 Months Ended				12 Months Ended		102 Months Ended
	Jan. 31, 2019	Apr. 30, 2016	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2016	Dec. 31, 2005	Mar. 31, 2020	Dec. 31, 2019
Loss Contingencies [Line Items]
Other current liabilities			$ 16,563,000						$ 16,563,000	$ 18,324,000
Other liabilities			11,773,000						11,773,000	$ 22,582,000
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 (Formerly Referred to as Guadalupe River Watershed) [Member] | Natural Resource Damage Claim [Member]
Loss Contingencies [Line Items]
Total reserve			1,500,000						1,500,000
Other current liabilities			300,000						300,000
Other liabilities			1,200,000						1,200,000
Expense recognized			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
Pending Litigation [Member] | New Idria Mercury Mine [Member] | EPA Notice Letter [Member]
Loss Contingencies [Line Items]
Loss contingencies, payments									3,300,000
Loss contingency, Loss in period			0	$ 4,000,000	$ 0				9,900,000
Total reserve			6,600,000						6,600,000
Other current liabilities			1,200,000						1,200,000
Other liabilities			$ 5,400,000						$ 5,400,000