Segment Reporting (Details 3) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	$ 709,598	$ 568,617	$ 554,557
China National Gold [Domain]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	137,500	126,600	126,200
Republic Metals Corporation [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	132,400	47,300	600
Asahi Formerly Johnson Matthey [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	124,100	62,600	84,200
Techemet Metal Trading [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	104,800	40,700	0
TD Securities [Domain]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	0	15,500	81,300
INTL Commodities [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	9,600	76,600	33,100
Mitsui & Co. [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	$ 0	$ 0	$ 137,700
Dore [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Number of Trading Counterparties	8
Dore [Member] | Sales [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Concentration Risk, Percentage	78.00%	74.00%	72.00%
Smelting and Refining [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Number of Trading Counterparties	1
Smelting and Refining [Member] | Sales [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Concentration Risk, Percentage	22.00%	26.00%	27.00%