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Financial instruments and risk management - Gains or Losses Associated with Ten Percent Strengthening of US Dollar (Detail)
 12 Months Ended
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Disclosure of detailed information about hedging instruments [Line Items]      
Effect on net earnings $ 56,000,000 $ (61,000,000) $ (38,000,000)
Australian dollar [Member]      
Disclosure of detailed information about hedging instruments [Line Items]      
Closing exchange rate 0.70 0.78 0.72
Canadian dollar [Member]      
Disclosure of detailed information about hedging instruments [Line Items]      
Closing exchange rate 0.73 0.79 0.74
Euro Member Countries, Euro      
Disclosure of detailed information about hedging instruments [Line Items]      
Closing exchange rate 1.14 1.20 1.05
Foreign exchange risk [member] | Australian dollar [Member]      
Disclosure of detailed information about hedging instruments [Line Items]      
Closing exchange rate 0.70 0.78  
Effect on net earnings $ 346,000,000 $ 608,000,000  
Amount impacting underlying earnings 1,000,000 47,000,000  
Foreign Exchange Gain Loss Amount Impact Directly On Equity $ (993,000,000) $ (1,657,000,000)  
Foreign exchange risk [member] | Canadian dollar [Member]      
Disclosure of detailed information about hedging instruments [Line Items]      
Closing exchange rate 0.73 0.79  
Effect on net earnings $ (82,000,000) $ (134,000,000)  
Amount impacting underlying earnings $ 7,000,000 $ 3,000,000  
Foreign exchange risk [member] | Euro Member Countries, Euro      
Disclosure of detailed information about hedging instruments [Line Items]      
Closing exchange rate 1.14 1.20  
Effect on net earnings $ 202,000,000 $ 165,000,000  
Amount impacting underlying earnings $ 5,000,000 $ (2,000,000)