Segment Reporting - Summary of Concentration Risk (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Concentration Risk [Line Items]
Revenue	$ 195,040	$ 199,469	$ 162,123	$ 154,870	$ 143,855	$ 148,795	$ 169,987	$ 163,267	$ 711,502	$ 625,904	$ 709,598
Customer Concentration Risk [Member] | Asahi Formerly Johnson Matthey [Member]
Concentration Risk [Line Items]
Revenue									341,000	213,000	124,100
Customer Concentration Risk [Member] | Techemet Metal Trading [Member]
Concentration Risk [Line Items]
Revenue									9,400	83,300	104,800
Customer Concentration Risk [Member] | Ocean Partners [Member]
Concentration Risk [Line Items]
Revenue									149,700	74,800	0
Customer Concentration Risk [Member] | RMC [Member]
Concentration Risk [Line Items]
Revenue									0	71,700	132,400
Customer Concentration Risk [Member] | China National Gold [Domain]
Concentration Risk [Line Items]
Revenue									$ 0	$ 54,100	$ 137,500
Dore [Member] | Product Concentration Risk [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage									68.00%	77.00%	78.00%
Concentrate [Member] | Product Concentration Risk [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage									32.00%	23.00%	22.00%