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Shares - Voting common shares (Details) - CAD
CAD in Thousands
 12 Months Ended
Aug. 14, 2017
May 25, 2017
Mar. 28, 2017
Aug. 09, 2016
Mar. 21, 2016
Dec. 31, 2017
Dec. 31, 2016
Increase (Decrease) in Stockholders' Equity [Roll Forward]              
Beginning balance, issued and outstanding (in shares)           30,518,907 33,150,281
Beginning balance, common share outstanding net of treasury shares (in shares)           28,305,660 31,893,478
Issued upon exercise of stock options (in shares)             102,040
Purchase of treasury shares for settlement of certain equity classified stock-based compensation (in shares)           353,592 87,554
Retired through share purchase programs (in shares) (1,142,762) (663,400) (819,395) (1,075,900) (1,657,514) (2,625,557.000) (2,733,414)
Ending balance, issued and outstanding (in shares)           28,070,150 30,518,907
Ending balance, common share outstanding net of treasury shares (in shares)           25,452,224 28,305,660
Equity classified stock-based compensation settled (in shares)           (161,285) (35,313)
Equity classified stock-based compensation settled           CAD 987 CAD 130
Share option plan              
Increase (Decrease) in Stockholders' Equity [Roll Forward]              
Issued upon exercise of stock options (in shares) [1]           176,800 102,040
Treasury shares              
Increase (Decrease) in Stockholders' Equity [Roll Forward]              
Beginning balance, issued and outstanding (in shares)           2,213,247 1,256,803
Purchase of treasury shares for settlement of certain equity classified stock-based compensation (in shares)           (758,271) (1,043,998)
Purchase of treasury shares for settlement of certain equity classified stock-based compensation (in shares)           353,592 87,554
Ending balance, issued and outstanding (in shares)           2,617,926 2,213,247
[1] All stock options exercised resulted in new common shares being issued (note 17(a))