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Financial instruments and risk management - Gains or Losses Associated with Ten Percent Strengthening of US Dollar (Detail)
$ in Millions
 12 Months Ended
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Disclosure of detailed information about hedging instruments [Line Items]      
Effect on net earnings $ (300) $ 52 $ 56
Australian dollar      
Disclosure of detailed information about hedging instruments [Line Items]      
Closing exchange rate 0.77 0.70 0.70
Canadian dollar      
Disclosure of detailed information about hedging instruments [Line Items]      
Closing exchange rate 0.78 0.77 0.73
Euro      
Disclosure of detailed information about hedging instruments [Line Items]      
Closing exchange rate 1.23 1.12 1.14
Foreign exchange risk | Australian dollar      
Disclosure of detailed information about hedging instruments [Line Items]      
Closing exchange rate 77 70  
Effect on net earnings $ 625 $ 453  
Amount impacting underlying earnings (11) (4)  
Impact directly on equity $ (1,105) $ (1,002)  
Foreign exchange risk | Canadian dollar      
Disclosure of detailed information about hedging instruments [Line Items]      
Closing exchange rate 78 77  
Effect on net earnings $ (167) $ (143)  
Amount impacting underlying earnings 6 7  
Impact directly on equity $ 0 $ 0  
Foreign exchange risk | Euro      
Disclosure of detailed information about hedging instruments [Line Items]      
Closing exchange rate 123 112  
Effect on net earnings $ 139 $ 178  
Amount impacting underlying earnings 3 4  
Impact directly on equity $ 0 $ 0