Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Total	Share Capital	Additional Paid in Capital	Deficit
Balance, beginning of year at Dec. 31, 2015		$ 10,186	$ 1,019,863	$ (28,407)
Increase (Decrease) in Stockholders' Equity
Issuance of Common Stock, net of share issuance costs (Note 7)		117	314,425
Exercise of stock options (Note 7)			5,347
Stock-based compensation (Note 7)			3,021
Net income (loss)	$ (465,565)			(465,565)
Balance, end of year at Dec. 31, 2016	858,987	10,303	1,342,656	(493,972)
Increase (Decrease) in Stockholders' Equity
Cumulative adjustment for accounting changes related to tax reorganizations				124,476
Repurchase of Common Stock (Note 7)		(8)	(17,908)
Stock-based compensation (Note 7)			2,496
Net income (loss)	(31,708)			(31,708)
Balance, end of year at Dec. 31, 2017	936,335	10,295	1,327,244	(401,204)
Increase (Decrease) in Stockholders' Equity
Repurchase of Common Stock (Note 7)		(5)	(12,737)
Exercise of stock options (Note 7)			1,429
Stock-based compensation (Note 7)			2,112
Net income (loss)	102,616			102,616
Balance, end of year at Dec. 31, 2018	$ 1,029,750	$ 10,290	$ 1,318,048	$ (298,588)