INVESTMENT IN MINERA SANTA CRUZ S.A. ("MSC") - SAN JOSE MINE - Summary of Operating Results from 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
INVESTMENT IN MINERA SANTA CRUZ S.A. ("MSC") - SAN JOSE MINE
Net Sales	$ 26,896	$ 13,430	$ 102,360	$ 43,373
Other operating expenses	(21,782)	(11,549)	(59,726)	(35,999)
(Loss) income before income and mining taxes	(14,903)	(8,636)	(28,772)	(15,867)	$ 18,700
Net income	(13,290)	(8,072)	(23,881)	(12,800)
Income tax recovery	(1,613)	(564)	(4,891)	(3,067)
(Loss) from investment in MSC, net of amortization	$ (4,973)	$ (462)	$ (7,450)	$ (535)
Corporate Venture
INVESTMENT IN MINERA SANTA CRUZ S.A. ("MSC") - SAN JOSE MINE
Ownership interest (as a percent)	100.00%	100.00%	100.00%	100.00%
MSC
INVESTMENT IN MINERA SANTA CRUZ S.A. ("MSC") - SAN JOSE MINE
Net Sales	$ 47,814	$ 52,722	$ 150,748	$ 161,900
Cost of goods and services sold	(48,193)	(43,596)	(135,007)	(132,710)
Other operating expenses	(5,164)	(5,255)	(12,776)	(16,348)
Other Expenses	(4,106)	(2,449)	(12,360)	(5,115)
(Loss) income before income and mining taxes	(9,649)	1,422	(9,395)	7,727
Current & deferred taxes	(889)	(715)	(7,965)	(3,765)
Net income	$ (10,538)	$ 707	$ (17,360)	$ 3,962
Ownership interest (as a percent)	49.00%	49.00%	49.00%	49.00%
Net income (loss)	$ (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
(Loss) from investment in MSC, net of amortization	$ (4,973)	$ (462)	$ (7,450)	$ (535)