CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY shares in Thousands, $ in Thousands, $ in Millions	Common Stock USD ($) shares	Warrants USD ($)	Accumulated Deficit USD ($)	Flow Through Common Shares CAD ($)	Flow Through Common Shares USD ($)	At-the-market (“ATM”) USD ($) shares	USD ($)
Balance (in shares) at Dec. 31, 2017 | shares	337,051
Balance at Dec. 31, 2017	$ 1,444,056	$ 3,823	$ (926,606)				$ 521,273
Increase (Decrease) in Shareholders' Equity
Stock-based compensation (note 9)	203						203
Exercise of stock options (note 8)	$ 40						40
Exercise of stock options (note 8) (in shares) | shares	35
Shareholder distributions (note 8)	$ (1,686)						(1,686)
Net (loss) income			(5,211)				(5,211)
Balance at Mar. 31, 2018	$ 1,442,613	3,823	(931,817)				514,619
Balance (in shares) at Mar. 31, 2018 | shares	337,086
Balance (in shares) at Dec. 31, 2017 | shares	337,051
Balance at Dec. 31, 2017	$ 1,444,056	3,823	(926,606)				521,273
Increase (Decrease) in Shareholders' Equity
Sale of common stock (note 8)				$ 20.0	$ 14,900
Balance at Dec. 31, 2018	$ 1,457,422		(971,476)				485,946
Balance (in shares) at Dec. 31, 2018 | shares	344,560
Increase (Decrease) in Shareholders' Equity
Stock-based compensation (note 9)	$ 61						61
Sale of common stock (note 8)						$ 1,851
Sale of common stock (note 8) (in shares) | shares						1,010
Common stock issued in connection with registered direct offering (note 8)	$ 17,785						17,785
Common stock issued in connection with registered direct offering (note 8) (in shares) | shares	14,194
Exercise of stock options (note 8)	$ 223						223
Exercise of stock options (note 8) (in shares) | shares	222
Net (loss) income			(10,136)				(10,136)
Balance at Mar. 31, 2019	$ 1,477,342	2,467	$ (981,612)				498,197
Balance (in shares) at Mar. 31, 2019 | shares	359,986
Increase (Decrease) in Shareholders' Equity
Adoption of ASU 2016-01 (note 2)							100
Warrants issued in connection with registered direct offering (note 8)		$ 2,467					$ 2,467