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Post-employment benefit plans - Significant Assumptions (Details) - year
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Post-employment benefit obligations    
Discount rate 3.20% 2.60%
Rate of compensation increase 2.25% 2.25%
Cost of living indexation rate 1.60% 1.60%
Life expectancy at age 65 (years) 23.3 23.2
Net post-employment benefit plans cost    
Discount rate 2.90% 3.20%
Rate of compensation increase 2.25% 2.25%
Cost of living indexation rate 1.60% 1.60%
Life expectancy at age 65 (years) 23.2 23.2
Weighted average duration of defined benefit obligation 14 years  
Cost of medication    
Net post-employment benefit plans cost    
Actuarial assumption of medical cost trend rates 6.50%  
Ultimate actuarial assumption of medical cost trend rates 4.00%  
Term to reach ultimate actuarial assumption of medical cost trend rate 20 years  
Cost of covered dental benefits    
Net post-employment benefit plans cost    
Actuarial assumption of medical cost trend rates 4.00%  
Cost of covered hospital benefits    
Net post-employment benefit plans cost    
Actuarial assumption of medical cost trend rates 3.70%  
Cost of other covered healthcare benefits    
Net post-employment benefit plans cost    
Actuarial assumption of medical cost trend rates 4.00%