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Shareholders' capital - Additional Information (Detail)
$ / shares in Units, $ / shares in Units, $ in Thousands
 1 Months Ended 2 Months Ended 3 Months Ended 12 Months Ended 23 Months Ended
Jul. 17, 2020
USD ($)
$ / shares
shares
Jul. 17, 2020
CAD ($)
shares
Mar. 31, 2019
shares
Mar. 30, 2019
$ / shares
Oct. 31, 2019
USD ($)
$ / shares
shares
Mar. 04, 2021
shares
Dec. 31, 2020
USD ($)
shares
Dec. 31, 2020
USD ($)
right
vote
$ / shares
shares
Dec. 31, 2020
USD ($)
$ / shares
shares
Dec. 31, 2019
USD ($)
shares
Dec. 31, 2019
USD ($)
$ / shares
Dec. 31, 2020
USD ($)
$ / shares
shares
Dec. 31, 2020
CAD ($)
$ / shares
shares
Jul. 17, 2020
$ / shares
Stockholders Equity Note [Line Items]                            
Number of entitled votes per common share | vote               1            
Number of voting rights per share | right               1            
Discount rate on share purchases (percent)               50.00%            
Discount rate on share purchases under dividend reinvestment plan (percent)               5.00%            
Carrying amount | $             $ 184,299,000 $ 184,299,000 $ 184,299,000 $ 184,299,000 $ 184,299,000 $ 184,299,000    
Share-based compensation expense | $               24,637,000   $ 11,042,000        
Unrecognized compensation costs, non-vested awards | $             $ 12,063,000 $ 12,063,000 $ 12,063,000     $ 12,063,000    
Unrecognized compensation costs, non-vested options, period of recognition               1 year 8 months 15 days            
Succession and retirement expense | $               $ 2,582,000            
Options granted (in shares)               999,962   1,113,775        
Options granted (CAD per share) | $ / shares                 $ 16.78   $ 14.96      
Options exercised (in shares)               2,386,275   3,882,505        
Options exercised (CAD per share) | $ / shares                 $ 12.52   11.23      
Total share-based compensation expense | $               $ 24,637,000   $ 11,042,000        
Company Match, First $5,000 Contributed by Employee                            
Stockholders Equity Note [Line Items]                            
Employer matching contribution (percent)               20.00%            
Company Match, Employee Contributions $5,001 to $10,000                            
Stockholders Equity Note [Line Items]                            
Employer matching contribution (percent)               10.00%            
Minimum                            
Stockholders Equity Note [Line Items]                            
Percentage of outstanding stock to be purchased to acquire discount (or more)               20.00%            
Common shares                            
Stockholders Equity Note [Line Items]                            
Options settled at cash value for payment of the exercise price and for tax withholdings (in shares)             7,377              
Treasury Stock, Common                            
Stockholders Equity Note [Line Items]                            
Shares issued from treasury for option exercise (in shares)               6,401,000            
Retirement Restricted Share Units                            
Stockholders Equity Note [Line Items]                            
Awards vested (percent)               100.00%            
Performance Share Units                            
Stockholders Equity Note [Line Items]                            
Maximum aggregate number of shares reserved for future issuance (in shares)             7,000,000 7,000,000 7,000,000     7,000,000 7,000,000  
Award vesting period               3 years            
Performance Share Units | Minimum                            
Stockholders Equity Note [Line Items]                            
Percentage of shares issued on number of PSU grants (percent)               2.50%            
Performance Share Units | Maximum                            
Stockholders Equity Note [Line Items]                            
Percentage of shares issued on number of PSU grants (percent)               237.00%            
Bonus Deferral Restricted Stock Units                            
Stockholders Equity Note [Line Items]                            
Shares issued during period (in shares)               135,409            
Deferred Share Units                            
Stockholders Equity Note [Line Items]                            
Common stock, shares issued (in shares)             13,778,000 13,778,000 13,778,000     13,778,000 13,778,000  
Maximum aggregate number of shares reserved for future issuance (in shares)             1,000,000 1,000,000 1,000,000     1,000,000 1,000,000  
Shares issued during period (in shares)               544,493   460,418        
Employee Stock Option                            
Stockholders Equity Note [Line Items]                            
Share-based compensation expense | $               $ 1,743,000   $ 1,288,000        
Percentage of shares reserved under the plan (must not exceed)               8.00%            
Total share-based compensation expense | $               $ 1,743,000   $ 1,288,000        
ESPP                            
Stockholders Equity Note [Line Items]                            
Vesting period of matching contribution shares               1 year            
Maximum aggregate number of shares reserved for future issuance (in shares)             4,000,000 4,000,000 4,000,000     4,000,000 4,000,000  
Shares issued during period (in shares)               302,727   253,538        
Subsequent Event | Common shares                            
Stockholders Equity Note [Line Items]                            
Dividend reinvestment plan shares issued (in shares)           1,403,635                
ATM Equity Program                            
Stockholders Equity Note [Line Items]                            
Number of shares issued pursuant to public offering (in shares)               8,664,563       10,421,362    
Cash proceeds from issuance of shares | $               $ 120,634,000       $ 142,668,000    
Treasury atock, amount reserved for issuance under the plan | $             $ 500,000,000 $ 500,000,000 $ 500,000,000     $ 500,000,000    
Sale of stock, average price per share (in USD) | $ / shares               $ 13.92       $ 13.69    
Gross proceeds from sale of stock, net of commissions | $               $ 119,126,000       $ 140,885,000    
Sale of stock, other related costs | $               $ 1,346,000       $ 3,413,000    
Series A Preferred Stock                            
Stockholders Equity Note [Line Items]                            
Shares issued, price per share (USD and CAD per share) | $ / shares                         $ 25  
Preferred stock issued (in shares)             4,800,000 4,800,000 4,800,000     4,800,000 4,800,000  
Carrying amount             $ 100,463,000 $ 100,463,000 $ 100,463,000     $ 100,463,000 $ 116,546  
Dividend declared per preferred share (CAD per share) | (per share)               $ 1.2905     1.2905      
Preferred dividend rate reset period               5 years            
Basis spread on five-year Government of Canada Bond Yield (percent)               2.94%            
Series D Preferred Stock                            
Stockholders Equity Note [Line Items]                            
Shares issued, price per share (USD and CAD per share) | $ / shares                         $ 25  
Preferred stock issued (in shares)             4,000,000 4,000,000 4,000,000     4,000,000 4,000,000  
Carrying amount             $ 83,836,000 $ 83,836,000 $ 83,836,000     $ 83,836,000 $ 97,259  
Dividend declared per preferred share (CAD per share) | (per share)       $ 1.25       $ 1.2728     $ 1.2671      
Preferred dividend rate reset period               5 years            
Preferred dividend, initial period     5 years                      
Basis spread on five-year Government of Canada Bond Yield (percent)               3.28%            
Series B Preferred Stock                            
Stockholders Equity Note [Line Items]                            
Convertible preferred stock, shares issued upon conversion     0                      
Series C Preferred Stock                            
Stockholders Equity Note [Line Items]                            
Redeemable preferred stock issued (in shares)             100 100 100     100 100  
Public Stock Offering | Common shares                            
Stockholders Equity Note [Line Items]                            
Number of shares issued pursuant to public offering (in shares) 57,465,500 57,465,500     26,252,542                  
Shares issued, price per share (USD and CAD per share) | (per share) $ 12.60       $ 13.50                 $ 17.10
Cash proceeds from issuance of shares $ 723,926,000 $ 982,660     $ 354,409,000                  
Issuance costs $ 25,268,000 $ 34,299     $ 14,418,000                  
Other Net Losses                            
Stockholders Equity Note [Line Items]                            
Share-based compensation expense | $               $ 12,639,000            
Total share-based compensation expense | $               12,639,000            
Other Net Losses | Retirement Restricted Share Units                            
Stockholders Equity Note [Line Items]                            
Share-based compensation expense | $               5,466,000            
Total share-based compensation expense | $               5,466,000            
Other Net Losses | Performance Share Units                            
Stockholders Equity Note [Line Items]                            
Share-based compensation expense | $               4,591,000            
Total share-based compensation expense | $               $ 4,591,000