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Post-retirement benefits - Additional information (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Disclosure of defined benefit plans [Line Items] |  |  |
Securities included in fair value of plan assets | $ 13 | $ 16 |
Defined benefit expenses excluded form unerlying earnings | 0 | $ 0 | $ 0
Estimated contributions to defined benefit pension plans for 2018 | $ 210 |  |
Defined benefit obligation weighted average expected future life time | 27 years | 26 years |
Defined benefit obligation weighted average expected future life time at man aged 60 in 2036 | 28 years | 28 years |
United States [member] |  |  |
Disclosure of defined benefit plans [Line Items] |  |  |
Discount rate | 3.50% | 3.90% |
Canada [member] |  |  |
Disclosure of defined benefit plans [Line Items] |  |  |
Discount rate | 3.40% | 3.80% |
Healthcare plans [member] | United States [member] |  |  |
Disclosure of defined benefit plans [Line Items] |  |  |
Discount rate | 3.60% | 3.90% |
Medical trend rate | 9.80% | 9.10% |
Reduction in medical trend rate | 4.70% | 5.10% |
Healthcare plans [member] | Canada [member] |  |  |
Disclosure of defined benefit plans [Line Items] |  |  |
Discount rate | 3.60% | 3.90% |
Medical trend rate | 9.80% | 9.10% |
Reduction in medical trend rate | 4.70% | 5.10% |

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- Definition The discount rate used as the significant actuarial assumption to determine the present value of a defined benefit obligation. [Refer: Defined benefit obligation, at present value; Actuarial assumptions [member]] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 19 -IssueDate 2016-01-13 -Paragraph 144 -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_144&doctype=Standard -URIDate 2016-03-31 + Details
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- Definition Medical cost trend rate used as significant actuarial assumption to determine the present value of a defined benefit obligation. [Refer: Defined benefit obligation, at present value; Actuarial assumptions [member]] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 19 -IssueDate 2016-01-13 -Paragraph 144 -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_144&doctype=Standard -URIDate 2016-03-31 + Details
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- Definition The fair value of the entity's own transferable financial instruments that are included in the fair value of defined benefit plan assets. [Refer: Plan assets, at fair value; Financial instruments, class [member]; Defined benefit plans [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2016-01-13 -Paragraph 143 -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_143&doctype=Standard -URIDate 2016-03-31 + Details
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- Definition Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table. + References No definition available. + Details
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- Definition The reasonably possible percentage of the decrease in the actuarial assumption used to determine the present value of defined benefit obligation. [Refer: Actuarial assumptions [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2016-01-13 -Paragraph 145 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_145_a&doctype=Standard -URIDate 2016-03-31 + Details
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- Definition Defined benefit expenses exlcuded from underlying earnings. + References No definition available. + Details
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- Definition Defined benefit obligation weighted average expected future life After Balance Sheet Date + References No definition available. + Details
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