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Post-retirement benefits - Summary of maturity of defined benefit obligations (Detail)
12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Disclosure of defined benefit plans [Line Items]      
Proportion relating to current employees 17.00% 18.00% 20.00%
Proportion relating to former employees not yet retired 9.00% 9.00% 11.00%
Proportion relating to retirees 74.00% 73.00% 69.00%
Total 100.00% 100.00% 100.00%
Average duration of obligations (years) 10 years 9 months 18 days 11 years 4 months 24 days 13 years 9 months 18 days
Pension benefits      
Disclosure of defined benefit plans [Line Items]      
Proportion relating to current employees 17.00%    
Proportion relating to former employees not yet retired 9.00%    
Proportion relating to retirees 74.00%    
Total 100.00%    
Average duration of obligations (years) 10 years 9 months 18 days    
Other benefits      
Disclosure of defined benefit plans [Line Items]      
Proportion relating to current employees 15.00%    
Proportion relating to former employees not yet retired 0.00%    
Proportion relating to retirees 85.00%    
Total 100.00%    
Average duration of obligations (years) 10 years 9 months 18 days