INCOME AND MINING TAXES - Reconciliation of Tax Provision (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
INCOME AND MINING TAXES.
(Loss) income before income and mining taxes	$ (47,640)	$ (26,003)	$ 17,306
Statutory tax rate (as a percent)	21.00%	35.00%	34.00%
US Federal and State tax (expense) recovery at statutory rate	$ (10,004)	$ (9,101)	$ 5,884
Reconciling items:
Equity pickup in MSC	2,966	(16)	(4,533)
Deferred foreign income inclusion	5,963	21,002
Foreign tax credits		(16,628)
Tax rate changes		28,048
Realized flow-through expenditures	2,100
Realized flow-through premium	(1,675)
Adjustment for foreign tax rates	40	115	(501)
Other permanent differences	4,419	(1,761)	818
Unrealized foreign exchange rate (loss)/gain	(6,935)	2,469	5,972
NOL expires and revisions	(120)	(2,806)	(242)
Valuation allowance	476	(36,691)	(11,147)
Tax benefit	$ (2,770)	$ (15,369)	$ (3,749)