COMMITMENTS AND CONTINGENCIES - Surety, Reclamation Bonds and Flow-Through Common Shares (Details) $ in Thousands, $ in Millions				3 Months Ended			12 Months Ended
	Mar. 29, 2019 USD ($)	Dec. 20, 2018 USD ($)	Dec. 19, 2017 USD ($)	Mar. 31, 2019 CAD ($)	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2018 CAD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 CAD ($)	Dec. 31, 2017 USD ($)
Reclamation Bonds
Other assets					$ 4,173			$ 5,872
Amount raised or spent in connection with the equity issuance	$ 20,300
Reclamation Bonds
Reclamation Bonds
Reclamation bonding obligation					$ 19,900
Black Fox | Surety Bonds
Reclamation Bonds
Percentage of annual fees on surety bonds				2.00%	2.00%
Percentage of upfront deposit on surety bonds				0.00%	0.00%
Surety bonding obligation				$ 20.6	$ 15,400
Outstanding surety bonds					20,000
Black Fox | Reclamation Bonds
Reclamation Bonds
Surety bonding obligation				$ 20.6	15,400
Timmins | Reclamation Bonds
Reclamation Bonds
Surety bonds upfront deposit amount					100
Flow Through Common Shares
Reclamation Bonds
Amount raised or spent in connection with the equity issuance		$ 14,900	$ 10,000				$ 20.0	$ 14,900	$ 12.9	$ 10,000
Total costs incurred for Canadian Exploration Expenditures					$ 400	$ 1,200