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Post-employee benefit plans - Healthcare Cost Trend Rates and Sensitivity Analysis (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2020
CAD ($)
Actuarial assumption of medical cost trend rates  
Disclosure of sensitivity analysis for actuarial assumptions [line items]  
Percentage of reasonably possible increase in actuarial assumption 1.00%
Percentage of reasonably possible decrease in actuarial assumption 1.00%
Impact on net post-employment benefit plans cost, increase in assumption $ 3
Impact on net post-employment benefit plans cost, decrease in assumption (3)
Impact on post-employment benefit obligation, increase in assumption 110
Impact on post-employment benefit obligation, decrease in assumption $ (95)
Actuarial assumption of discount rates  
Disclosure of sensitivity analysis for actuarial assumptions [line items]  
Percentage of reasonably possible increase in actuarial assumption 0.50%
Percentage of reasonably possible decrease in actuarial assumption 0.50%
Impact on net post-employment benefit plans cost, increase in assumption $ (76)
Impact on net post-employment benefit plans cost, decrease in assumption 64
Impact on post-employment benefit obligation, increase in assumption (1,897)
Impact on post-employment benefit obligation, decrease in assumption 2,127
Actuarial assumption of life expectancy  
Disclosure of sensitivity analysis for actuarial assumptions [line items]  
Impact on net post-employment benefit plans cost, increase in assumption 38
Impact on net post-employment benefit plans cost, decrease in assumption (38)
Impact on post-employment benefit obligation, increase in assumption 1,092
Impact on post-employment benefit obligation, decrease in assumption $ (1,092)
Duration of reasonably possible decrease in actuarial assumptions 1 year
Duration of reasonably possible increase in actuarial assumption 1 year