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Post-retirement benefits - Results summary of sensitivity analysis (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Discount rate | Pension benefits    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Increase of percentage points 50.00% 50.00%
Decrease of percentage points 50.00% 50.00%
Increase of actuarial assumption $ 854 $ 988
Decrease of actuarial assumption $ (915) $ (1,186)
Discount rate | Other benefits    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Increase of percentage points 50.00% 50.00%
Decrease of percentage points 50.00% 50.00%
Increase of actuarial assumption $ 51 $ 62
Decrease of actuarial assumption $ (55) $ (66)
Inflation | Pension benefits    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Increase of percentage points 50.00% 50.00%
Decrease of percentage points 50.00% 50.00%
Increase of actuarial assumption $ (393) $ (484)
Decrease of actuarial assumption $ 374 $ 450
Inflation | Other benefits    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Increase of percentage points 50.00% 50.00%
Decrease of percentage points 50.00% 50.00%
Increase of actuarial assumption $ (14) $ (19)
Decrease of actuarial assumption $ 13 $ 17
Salary increases | Pension benefits    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Increase of percentage points 50.00% 50.00%
Decrease of percentage points 50.00% 50.00%
Increase of actuarial assumption $ (55) $ (81)
Decrease of actuarial assumption $ 54 $ 72
Salary increases | Other benefits    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Increase of percentage points 50.00% 50.00%
Decrease of percentage points 50.00% 50.00%
Increase of actuarial assumption $ (1) $ (1)
Decrease of actuarial assumption $ 1 $ 1
Demographic – allowance for future improvements in longevity | Pension benefits    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Increase in actuarial assumption of longevity trend 1 year 1 year
Decrease in actuarial assumption of longevity trend 1 year 1 year
Increase of actuarial assumption $ 441 $ 520
Decrease of actuarial assumption $ (441) $ (562)
Demographic – allowance for future improvements in longevity | Other benefits    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Increase in actuarial assumption of longevity trend 1 year 1 year
Decrease in actuarial assumption of longevity trend 1 year 1 year
Increase of actuarial assumption $ 15 $ 19
Decrease of actuarial assumption $ (15) $ (19)