INVESTMENT IN MINERA SANTA CRUZ S.A. ("MSC") - SAN JOSE MINE - Changes in Company's Investment in MSC (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Change in the investment in MSC
Investment in MSC, beginning balance			$ 150,064
Income tax recovery	$ (1,613)	$ (564)	(4,891)	$ (3,067)
Distributions received			(9,360)	(7,195)
Investment in MSC, ending balance	133,254		133,254		$ 150,064
MSC
Change in the investment in MSC
Investment in MSC, beginning balance			150,064	162,320	162,320
Attributable net income (loss) from MSC	(5,164)	346	(8,506)	1,941	(2,328)
Amortization of fair value increments	(2,538)	(2,469)	(7,135)	(6,967)	(9,632)
Income tax recovery	2,728	$ 1,661	8,191	$ 4,491	11,916
Distributions received			(9,360)		(12,212)
Investment in MSC, ending balance	$ 133,254		$ 133,254		$ 150,064