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Financial instruments and risk management - Gains or Losses Associated with Ten Percent Strengthening of US Dollar (Detail) | 12 Months Ended
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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) |  |

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