Capital Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Stock-based compensation disclosure
Aggregate number of common shares reserved for future issuance	25,000,000.0
Proceeds from issuance of common stock under stock plans	$ 1,471	$ 2,873	$ 4,301
Stock options
Stock-based compensation disclosure
Stock options granted to directors (in shares)	7,421,000
Options exercised (in shares)	336,000
Exercise price (in dollars per share)	$ 2.31
Exercise price (in dollars per share)	$ 5.25
Proceeds from issuance of common stock under stock plans	$ 1,000
Exercisable at the end of the period (in shares)	6,983,000	5,801,000	8,405,000
Weighted-Average Exercise Price (in dollars per share)	$ 8.15	$ 10.16	$ 11.47
2018 Plan | Stock options
Stock-based compensation disclosure
Options exercised (in shares)	336,000
Compensation Policy for Non-Employee Directors | Stock options
Stock-based compensation disclosure
Stock options granted to directors (in shares)	300,000	108,000	128,000
Compensation Policy for Non-Employee Directors | Deferred share units
Stock-based compensation disclosure
Compensation expense	$ 300	$ 300	$ 400
Common stock issued to retiring directors (in shares)	127,000	63,000	46,000
Common stock issued when a director ceases to be a member (in shares)	1
Stock units previously granted (in shares)	15,000	18,000	10,500
Compensation Policy for Non-Employee Directors | Deferred share units | Retiring director
Stock-based compensation disclosure
Common stock issued to retiring directors (in shares)			172,509