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Post-retirement benefits - Summary of maturity of defined benefit obligations (Detail)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of defined benefit plans [Line Items]      
Proportion relating to current employees 21.00% 20.00% 19.00%
Proportion relating to former employees not yet retired 11.00% 12.00% 11.00%
Proportion relating to retirees 68.00% 68.00% 70.00%
Total 100.00% 100.00% 100.00%
Average duration of obligations (years) 14 years 3 months 18 days 14 years 4 months 24 days 13 years 4 months 24 days
Pension benefits      
Disclosure of defined benefit plans [Line Items]      
Proportion relating to current employees 21.00%    
Proportion relating to former employees not yet retired 12.00%    
Proportion relating to retirees 67.00%    
Total 100.00%    
Average duration of obligations (years) 14 years 4 months 24 days    
Other Benefits      
Disclosure of defined benefit plans [Line Items]      
Proportion relating to current employees 19.00%    
Proportion relating to former employees not yet retired 0.00%    
Proportion relating to retirees 81.00%    
Total 100.00%    
Average duration of obligations (years) 13 years 6 months