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Shares - Share purchase program (Details) - CAD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Aug. 14, 2017
Aug. 09, 2016
Equity, Class of Treasury Stock [Line Items]        
Shares purchased and subsequently cancelled during period (in shares) (1,281,485) (2,625,557)    
2017 Share Repurchase Program        
Equity, Class of Treasury Stock [Line Items]        
Maximum number of shares to be purchased (in shares)     2,424,333  
Shares purchased and subsequently cancelled during period (in shares) (1,281,485.000) (1,142,762)    
2016 Share Repurchase Program        
Equity, Class of Treasury Stock [Line Items]        
Maximum number of shares to be purchased (in shares)       2,733,482
Shares purchased and subsequently cancelled during period (in shares)   (1,657,514)    
Common shares        
Equity, Class of Treasury Stock [Line Items]        
Shares purchased and subsequently cancelled during period (in shares) (1,281,485) (2,625,557)    
Common shares | 2017 Share Repurchase Program        
Equity, Class of Treasury Stock [Line Items]        
Increase (decrease) as a result of the retirement of shares $ (10,975) $ (9,810)    
Common shares | 2016 Share Repurchase Program        
Equity, Class of Treasury Stock [Line Items]        
Shares purchased and subsequently cancelled during period (in shares)   (1,482,795)    
Increase (decrease) as a result of the retirement of shares   $ (12,763)    
Additional paid-in capital | 2017 Share Repurchase Program        
Equity, Class of Treasury Stock [Line Items]        
Increase (decrease) as a result of the retirement of shares $ 1,435 4,119    
Additional paid-in capital | 2016 Share Repurchase Program        
Equity, Class of Treasury Stock [Line Items]        
Increase (decrease) as a result of the retirement of shares   $ 3,484