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Post-retirement benefits - Summary of maturity of defined benefit obligations (Detail)
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Disclosure of defined benefit plans [Line Items]      
Proportion relating to current employees 20.00% 21.00% 20.00%
Proportion relating to former employees not yet retired 11.00% 11.00% 12.00%
Proportion relating to retirees 69.00% 68.00% 68.00%
Total 100.00% 100.00% 100.00%
Average duration of obligations (years) 13 years 9 months 18 days 14 years 3 months 18 days 14 years 4 months 24 days
Pension benefits      
Disclosure of defined benefit plans [Line Items]      
Proportion relating to current employees 20.00%    
Proportion relating to former employees not yet retired 12.00%    
Proportion relating to retirees 68.00%    
Total 100.00%    
Average duration of obligations (years) 13 years 9 months 18 days    
Other benefits      
Disclosure of defined benefit plans [Line Items]      
Proportion relating to current employees 17.00%    
Proportion relating to former employees not yet retired 0.00%    
Proportion relating to retirees 83.00%    
Total 100.00%    
Average duration of obligations (years) 13 years 1 month 6 days