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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, 2019
USD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Disclosure of detailed information about hedging instruments [Line Items]      
Effect on net earnings $ 52 $ 56 $ (61)
Australian dollar      
Disclosure of detailed information about hedging instruments [Line Items]      
Closing exchange rate 0.70 0.70 0.78
Canadian dollar      
Disclosure of detailed information about hedging instruments [Line Items]      
Closing exchange rate 0.77 0.73 0.79
Euro      
Disclosure of detailed information about hedging instruments [Line Items]      
Closing exchange rate 1.12 1.14 1.20
Foreign exchange risk | Australian dollar      
Disclosure of detailed information about hedging instruments [Line Items]      
Closing exchange rate 70 70  
Effect on net earnings $ 453 $ 346  
Amount impacting underlying earnings (4) 1  
Foreign Exchange Gain Loss Amount Impact Directly On Equity $ (1,002) $ (993)  
Foreign exchange risk | Canadian dollar      
Disclosure of detailed information about hedging instruments [Line Items]      
Closing exchange rate 77 73  
Effect on net earnings $ (143) $ (82)  
Amount impacting underlying earnings 7 7  
Foreign Exchange Gain Loss Amount Impact Directly On Equity $ 0 $ 0  
Foreign exchange risk | Euro      
Disclosure of detailed information about hedging instruments [Line Items]      
Closing exchange rate 112 114  
Effect on net earnings $ 178 $ 202  
Amount impacting underlying earnings 4 5  
Foreign Exchange Gain Loss Amount Impact Directly On Equity $ 0 $ 0