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Pension and other post-retirement benefits - components of net pension and other benefit expense (Details) - CAD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Disclosure of benefit plans [line items]    
Defined benefit expense $ 5,256 $ 4,790
Defined contribution pension expense 12,410 11,767
Employee benefit expense 322,669 312,561
Pension plans [member]    
Disclosure of benefit plans [line items]    
Current service cost 1,977 1,586
Interest income or cost 1,476 1,545
Administration cost 1 1
Defined benefit expense 3,454 3,132
Defined contribution pension expense 12,410 11,767
Employee benefit expense 15,864 14,899
Amount of actuarial losses (gains) recognized in other comprehensive income    
Actuarial loss (gain) 6,673 7,702
Return on assets excluding interest income (130) (280)
Total actuarial losses (gains) recognized in other comprehensive income 6,543 7,422
Other Benefit Plans [Member]    
Disclosure of benefit plans [line items]    
Current service cost 1,010 817
Interest income or cost 792 841
Past service cost 0 0
Administration cost 0 0
Defined benefit expense 1,802 1,658
Defined contribution pension expense 0 0
Employee benefit expense 1,802 1,658
Amount of actuarial losses (gains) recognized in other comprehensive income    
Actuarial loss (gain) (121) 2,991
Return on assets excluding interest income 0 0
Total actuarial losses (gains) recognized in other comprehensive income $ (121) $ 2,991