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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, 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) |

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