INCOME AND MINING TAXES - Reconciliation of Tax Provision (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
INCOME AND MINING TAXES.
Loss before income and mining taxes	$ (63,591)	$ (47,640)	$ (26,003)
Statutory tax rate (as a percent)	21.00%	21.00%	35.00%
US Federal and State tax recovery (expense) at statutory rate	$ (13,354)	$ (10,004)	$ (9,101)
Reconciling items:
Equity pickup in MSC	2,626	2,966	(16)
Deferred foreign income inclusion	598	5,963	21,002
Realized flow-through expenditures	3,150	2,100
Realized flow-through premium	(2,954)	(1,675)
Foreign tax credits			(16,628)
Tax rate changes	976		28,048
Adjustment for foreign tax rates	(200)	40	115
Other permanent differences	8,540	4,419	(1,761)
Unrealized foreign exchange rate (loss)/gain	(1,095)	(6,935)	2,469
NOL expires and revisions	810	(120)	(2,806)
Valuation allowance	(2,941)	476	(36,691)
Tax benefit	$ (3,844)	$ (2,770)	$ (15,369)