Capital Stock (Details) - USD ($)			1 Months Ended			6 Months Ended	12 Months Ended
	Mar. 28, 2018	Mar. 27, 2018	Jun. 30, 2018	Feb. 28, 2018	Jan. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2001
Stock-based compensation disclosure
Compensation expense						$ 8,100,000	$ 16,400,000	$ 11,100,000	$ 21,900,000
Aggregate number of common shares reserved for future issuance							24,700,000
Proceeds from issuance of common stock under stock plans							$ 4,301,000	$ 650,000	$ 5,161,000
Stock options
Stock-based compensation disclosure
Stock options granted to directors (in shares)						3,536,000	5,513,000	1,589,000	3,340,000
Options exercised (in shares)							742,000	191,000	555,000
Exercise price (in dollars per share)							$ 1.84
Exercise price (in dollars per share)							$ 12.21
Proceeds from issuance of common stock under stock plans							$ 3,500,000
Exercisable at the end of the period (in shares)						7,898,000	8,405,000	7,996,000	6,453,000
Weighted-Average Exercise Price (in dollars per share)						$ 13.15	$ 11.47	$ 12.16	$ 12.63
Deferred share units
Stock-based compensation disclosure
Compensation expense						$ 215,000	$ 361,000	$ 206,000	$ 380,000
2001 Non-Employee Director Stock Plan
Stock-based compensation disclosure
Aggregate number of common shares reserved for future issuance											50,000
2001 Non-Employee Director Stock Plan | Retiring director
Stock-based compensation disclosure
Compensation expense			$ 72,000
2001 Non-Employee Director Stock Plan | Deferred share units
Stock-based compensation disclosure
Compensation expense						$ (7,000)	$ 31,000	$ 28,000	$ (72,000)
Stock units outstanding (in shares)							6,000
2018 Plan | Stock options
Stock-based compensation disclosure
Options exercised (in shares)							742,000
Compensation Policy for Non-Employee Directors | Stock options
Stock-based compensation disclosure
Stock options granted to directors (in shares)						40,000	128,000	80,000	80,000	80,000
Compensation Policy for Non-Employee Directors | Deferred share units
Stock-based compensation disclosure
Compensation expense						$ 215,000	$ 361,000	$ 206,000	$ 380,000
Common stock issued to retiring directors (in shares)			95,497	77,012	53,248	37,000	46,000	47,000	41,000
Common stock issued when a director ceases to be a member (in shares)							1
Stock units previously granted (in shares)						12,000	10,500	12,000	12,000
Non-employee directors-initial grant | Deferred share units
Stock-based compensation disclosure
Vesting period							3 years
Stock options granted to directors (in shares)							6,500
Non-employee directors-after year one | Deferred share units
Stock-based compensation disclosure
Vesting period							1 year
Stock options granted to directors (in shares)							4,000
Annual stock option awards | Stock options
Stock-based compensation disclosure
Vesting period							1 year
Stock options granted to directors (in shares)	18,000	10,000
Off-Cycle initial awards | Stock options
Stock-based compensation disclosure
Stock options granted to directors (in shares)	18,000	10,000