CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Common Stock	Warrants	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit	Total
Balance at Dec. 31, 2016	$ 1,360,345		$ 1,666	$ (918,972)	$ 443,039
Balance (in shares) at Dec. 31, 2016	299,570
Increase (Decrease) in Shareholders' Equity
Stock-based compensation (note 9)	$ 330				330
Shareholders' distribution (note 8)	(1,498)				(1,498)
Exercise of stock options (note 8)	$ (47)				(47)
Exercise of stock options (note 8) (in shares)	20
Unrealized gain on available-for-sale securities, net of taxes (note 2)			3,875		3,875
Net loss				(3,018)	(3,018)
Balance at Mar. 31, 2017	$ 1,359,224		5,541	(921,990)	442,775
Balance (in shares) at Mar. 31, 2017	299,590
Increase (Decrease) in Shareholders' Equity
Adoption of ASU 2016-01 (note 2) | Accounting Standards Update 2016-01			(3,000)	3,000
Balance at Dec. 31, 2017	$ 1,444,056	$ 3,823	$ 3,000	(929,606)	521,273
Balance (in shares) at Dec. 31, 2017	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
Shareholders' distribution (note 8)	(1,686)				(1,686)
Exercise of stock options (note 8)	$ 40				40
Exercise of stock options (note 8) (in shares)	35
Net loss				(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	337,086