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Shareholders' capital - Additional Information (Detail)
 1 Months Ended 3 Months Ended 6 Months Ended
Jul. 01, 2019
USD ($)
$ / shares
shares
Mar. 31, 2019
$ / shares
shares
Jun. 30, 2019
USD ($)
$ / shares
shares
Jun. 30, 2018
USD ($)
Jun. 30, 2019
USD ($)
$ / shares
shares
Jun. 30, 2019
USD ($)
$ / shares
$ / shares
Jun. 30, 2018
USD ($)
Jun. 30, 2019
$ / shares
Feb. 28, 2019
USD ($)
Stockholders Equity Note [Line Items]                  
Options granted (CAD per share) | $ / shares           $ 14.96      
Exercise period of options (in years)         8 years        
Options exercised (USD per share) | $ / shares   $ 10.44              
Options settled (in shares)   2,022,382              
Share-based compensation expense | $     $ 2,566,000 $ 2,061,000 $ 4,473,000   $ 3,645,000    
Unrecognized compensation costs, non-vested options | $     1,808,000   1,808,000 $ 1,808,000      
Unrecognized compensation costs, non-vested awards | $     $ 9,993,000   $ 9,993,000 $ 9,993,000      
Stock Option Plans                  
Stockholders Equity Note [Line Items]                  
Common stock, shares issued (in shares)   573,975              
Unrecognized compensation costs, non-vested, period of recognition         2 years 1 month 6 days        
Performance share units                  
Stockholders Equity Note [Line Items]                  
Common stock, shares issued (in shares)   179,830              
Shares settled for stock (in shares)   344,340              
Shares settled for payments (in shares)   164,510              
Unrecognized compensation costs, non-vested, period of recognition         1 year 9 months 19 days        
Bonus Deferral Restricted Stock Units                  
Stockholders Equity Note [Line Items]                  
Shares granted (in shares)     125,001            
Deferred Share Units                  
Stockholders Equity Note [Line Items]                  
Shares issued during period (in shares)         42,215        
Executive                  
Stockholders Equity Note [Line Items]                  
Options granted (in shares)         1,113,775        
Options exercised (in shares)   2,596,357              
Executive | Performance share units                  
Stockholders Equity Note [Line Items]                  
Shares granted (in shares) 385,967 366,787              
Series D Preferred Stock                  
Stockholders Equity Note [Line Items]                  
Dividend payable (in CAD per share) | $ / shares               $ 1.2728  
Dividend rate, subsequent yield period         5 years        
Preferred stock redemption price per share (in CAD per share) | $ / shares               $ 25  
Subsequent redemption period         5 years        
Government of Canada Bond Yield, Five Year | Series D Preferred Stock                  
Stockholders Equity Note [Line Items]                  
Basis spread on variable rate (as a percent)         3.28%        
ATM Equity Program                  
Stockholders Equity Note [Line Items]                  
Sale of Stock, Value of Shares Issued in Transaction | $                 $ 250,000
Value of common shares issued from treasury         509,431        
Price per share of sale of stock (in usd per share) | $ / shares     $ 12.23   $ 12.23 $ 12.23      
Gross proceeds from sale of stock | $         $ 6,233,000        
Related commissions and fees on sale of stock | $         1,128,000        
Net proceeds from sale of stock | $         $ 6,139,000        
Subsequent Event | ATM Equity Program                  
Stockholders Equity Note [Line Items]                  
Value of common shares issued from treasury 564,500                
Price per share of sale of stock (in usd per share) | $ / shares $ 12.36                
Gross proceeds from sale of stock | $ $ 6,979,000                
Net proceeds from sale of stock | $ $ 6,874,000