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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, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Disclosure of detailed information about hedging instruments [Line Items]      
Effect on net earnings $ (61,000,000) $ (38,000,000) $ 263,000,000
Australian Dollars [Member]      
Disclosure of detailed information about hedging instruments [Line Items]      
Closing exchange rate 0.78 0.72 0.73
Canada, Dollars [Member]      
Disclosure of detailed information about hedging instruments [Line Items]      
Closing exchange rate 0.79 0.74 0.72
Euro Member Countries, Euro      
Disclosure of detailed information about hedging instruments [Line Items]      
Closing exchange rate 1.20 1.05 1.09
Foreign exchange risk [member] | Australian Dollars [Member]      
Disclosure of detailed information about hedging instruments [Line Items]      
Closing exchange rate 0.78 0.72  
Effect on net earnings $ 608,000,000 $ 369,000,000  
Amount impacting underlying earnings $ 47,000,000 $ 37,000,000  
Foreign exchange risk [member] | Canada, Dollars [Member]      
Disclosure of detailed information about hedging instruments [Line Items]      
Closing exchange rate 0.79 0.74  
Effect on net earnings $ (134,000,000) $ (159,000,000)  
Amount impacting underlying earnings $ 3,000,000 $ 15,000,000  
Foreign exchange risk [member] | Euro Member Countries, Euro      
Disclosure of detailed information about hedging instruments [Line Items]      
Closing exchange rate 1.20 1.05  
Effect on net earnings $ 165,000,000 $ 172,000,000  
Amount impacting underlying earnings $ (2,000,000)