Segment Reporting - Summary of Concentration Risk (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Concentration Risk [Line Items]
Revenue	$ 228,317	$ 229,728	$ 154,249	$ 173,167	$ 195,040	$ 199,469	$ 162,123	$ 154,870	$ 785,461	$ 711,502	$ 625,904
Customer Concentration Risk [Member] | Asahi Formerly Johnson Matthey [Member]
Concentration Risk [Line Items]
Revenue									272,100	341,000	213,000
Customer Concentration Risk [Member] | Techemet Metal Trading [Member]
Concentration Risk [Line Items]
Revenue									81,800	9,400	83,300
Customer Concentration Risk [Member] | Ocean Partners [Member]
Concentration Risk [Line Items]
Revenue									161,000	149,700	74,800
Customer Concentration Risk [Member] | RMC [Member]
Concentration Risk [Line Items]
Revenue									0	0	71,700
Customer Concentration Risk [Member] | China National Gold [Domain]
Concentration Risk [Line Items]
Revenue									0	0	54,100
Customer Concentration Risk [Member] | Toronto Dominion Bank
Concentration Risk [Line Items]
Revenue									88,600	35,100	44,900
Customer Concentration Risk [Member] | Argor-Heraeus
Concentration Risk [Line Items]
Revenue									$ 79,900	$ 23,100	$ 0
Dore [Member] | Product Concentration Risk [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage									72.00%	68.00%	77.00%
Concentrate [Member] | Product Concentration Risk [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage									28.00%	32.00%	23.00%