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Share-based payments - Additional Information (Detail)
$ in Thousands
 12 Months Ended
Nov. 01, 2015
Tranches
May 01, 2014
OptionPlans
Dec. 31, 2017
USD ($)
Options
shares
Dec. 31, 2016
USD ($)
shares
Options
Dec. 31, 2015
USD ($)
shares
Options
Dec. 31, 2017
CAD ($)
Options
Dec. 31, 2016
CAD ($)
shares
Options
Disclosure of terms and conditions of share-based payment arrangement [line items]              
Number of share option plans | OptionPlans   2          
Expense from share-based payment transactions     $ 11,218 $ 10,559      
Employee stock option plan [member]              
Disclosure of terms and conditions of share-based payment arrangement [line items]              
Term of employee option plan     5 years        
Description of option pricing model, share options granted     Generally, Employee Plan Options granted before November 1, 2015 vest in three equal and separate tranches with the first tranche vesting on the grant date and the second and third tranches vesting on the second and third anniversary dates of the grant date. Employee Plan Options granted on or after November 1, 2015 vest in three equal and separate tranches with vesting commencing one year after the date of grant and the second and third tranches vesting on the second and third anniversary of the grant date. Employee Plan Options are subject to withholding tax on exercise, withholding tax is paid by the Employee Option holder to the Company prior to receipt of the shares received pursuant to exercise.        
Number of tranches | Tranches 3            
Number of share options available to grant under the plan | shares     14,155,248 14,701,541      
Number of share purchase options vested and exercisable | Options     18,583,426 18,164,617   18,583,426 18,164,617
Weighted average exercise price of share purchase options vested and exercisable           $ 7.34 $ 9.64
Expense from share-based payment transactions     $ 6,736 $ 6,812      
Weighted average fair value per stock option           $ 1.53 $ 1.02
Directors and officers stock option plan [member]              
Disclosure of terms and conditions of share-based payment arrangement [line items]              
Term of employee option plan     5 years        
Description of option pricing model, share options granted     Generally, D&O Plan Options granted before November 1, 2015 vest in three equal and separate tranches with the first tranche vesting on the grant date and the second and third tranches vesting on the second and third anniversary dates of the grant date. D&O Plan Options granted on or after November 1, 2015 vest in three equal and separate tranches with vesting commencing one year after the date of grant and the second and third tranches vesting on the second and third anniversary of the grant date.        
Number of tranches | Tranches 3            
Number of share options available to grant under the plan | shares     3,720,125 4,243,018      
Restricted share unit plan [member]              
Disclosure of terms and conditions of share-based payment arrangement [line items]              
Description of option pricing model, share options granted     The RSUs vest as follows one third on the first anniversary of the grant date, one third on the second anniversary of the grant date and one third on the third anniversary of the grant date. RSUs terminate on the third anniversary of the grant date.        
Number of share purchase options vested and exercisable | shares       596,780 283,736   596,780
Vesting percentage on first anniversary     33.33%        
Vesting percentage on second anniversary     33.33%        
Vesting percentage on third anniversary     33.33%        
Maximum number of shares reserved for issue under options | Options     5,000,000     5,000,000  
Number of share options granted in share-based payment arrangement | Options     936,832 784,203      
Weighted average fair value per stock option           $ 4.49 $ 3.22
Number of share purchased held in trust share-based payment arrangement | shares       1,706,096 549,507    
Compensation expense       $ 2,716 $ 1,888    
Number of shares outstanding | Options     1,706,096 1,240,174 884,846 1,706,096 1,240,174
Deferred share units plans [member]              
Disclosure of terms and conditions of share-based payment arrangement [line items]              
Description of option pricing model, share options granted     Final receipt of the Redemption Notice is due fifteen (15) trading days after the Termination Date but no later than December 31 of the first calendar year commencing after the calendar year in which the Termination Date occurred (the "Redemption Date"), the Participant shall have the right to receive, and shall receive, with respect to all DUs held at the Redemption Date a cash payment equal to the market value of such DUs as of the Redemption Date.        
Trading days     15 days        
Number of shares outstanding | Options     596,836 498,390   596,836 498,390
Liabilities from share-based payment transactions     $ 866 $ 1,604      
Compensation expense (income)     $ 1,023 $ 295      
Performance share unit [member]              
Disclosure of terms and conditions of share-based payment arrangement [line items]              
Description of option pricing model, share options granted     PSUs cliff vest on the third anniversary of the grant date (the "Redemption Date") and are subject to terms and conditions including the achievement of predetermined performance criteria (the "Performance Criteria").        
Maximum number of shares reserved for issue under options | Options     3,130,000     3,130,000  
Number of share options granted in share-based payment arrangement | Options     569,719 796,652      
Compensation expense     $ 2,789 $ 1,564      
Performance share unit [member] | Bottom of range [member]              
Disclosure of terms and conditions of share-based payment arrangement [line items]              
Performance target award (range)     0.00%        
Performance share unit [member] | Top of range [member]              
Disclosure of terms and conditions of share-based payment arrangement [line items]              
Performance target award (range)     200.00%