Segment Reporting - Summary of Concentration Risk (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Concentration Risk [Line Items]
Revenue	$ 143,855	$ 148,795	$ 169,987	$ 163,267	$ 214,585	$ 159,919	$ 149,540	$ 185,554	$ 625,904	$ 709,598	$ 571,897
Customer Concentration Risk [Member] | Asahi Formerly Johnson Matthey [Member] | Sales Revenue, Net [Member]
Concentration Risk [Line Items]
Revenue									213,000	124,100	62,600
Customer Concentration Risk [Member] | Techemet Metal Trading [Member] | Sales Revenue, Net [Member]
Concentration Risk [Line Items]
Revenue									83,300	104,800	40,700
Customer Concentration Risk [Member] | Ocean Partners [Member] | Sales Revenue, Net [Member]
Concentration Risk [Line Items]
Revenue									74,800	0	0
Customer Concentration Risk [Member] | RMC [Member] | Sales Revenue, Net [Member]
Concentration Risk [Line Items]
Revenue									71,700	132,400	47,300
Customer Concentration Risk [Member] | China National Gold [Domain] | Sales Revenue, Net [Member]
Concentration Risk [Line Items]
Revenue									54,100	137,500	126,600
Customer Concentration Risk [Member] | INTL Commodities [Member] | Sales Revenue, Net [Member]
Concentration Risk [Line Items]
Revenue									$ 0	$ 9,600	$ 76,600