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Post-retirement benefits - Summary of amount recognised in balance sheet (Detail) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
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Disclosure of information about amounts recognized in balance sheet [Line Items] |  |  |
Net deficit to be shown in the balance sheet | $ (2,499) | $ (2,542) | $ (2,530)
Deficits | (3,370) | (3,167) |
Surpluses | 871 | 625 |
Net deficits on pension plans | (1,501) | (1,616) |
Unfunded [member] |  |  |
Disclosure of information about amounts recognized in balance sheet [Line Items] |  |  |
Unfunded post-retirement healthcare obligation | (998) | (926) |
Pension benefits [member] |  |  |
Disclosure of information about amounts recognized in balance sheet [Line Items] |  |  |
Net deficit to be shown in the balance sheet | (1,501) | (1,616) |
Deficits | (2,372) |  |
Surpluses | 871 |  |
Net deficits on pension plans | (1,501) |  |
Other Benefits [member] |  |  |
Disclosure of information about amounts recognized in balance sheet [Line Items] |  |  |
Net deficit to be shown in the balance sheet | (998) | (926) |
Deficits | (998) |  |
Other Benefits [member] | Unfunded [member] |  |  |
Disclosure of information about amounts recognized in balance sheet [Line Items] |  |  |
Unfunded post-retirement healthcare obligation | (998) |  |
Plan assets [Member] |  |  |
Disclosure of information about amounts recognized in balance sheet [Line Items] |  |  |
Net deficit to be shown in the balance sheet | 15,257 | 13,749 | 13,642
Plan assets [Member] | Pension benefits [member] |  |  |
Disclosure of information about amounts recognized in balance sheet [Line Items] |  |  |
Net deficit to be shown in the balance sheet | 15,257 | 13,749 |
Present value of defined benefit obligation [Member] |  |  |
Disclosure of information about amounts recognized in balance sheet [Line Items] |  |  |
Net deficit to be shown in the balance sheet | (17,645) | (16,228) | (16,123)
Present value of defined benefit obligation [Member] | Funded [member] |  |  |
Disclosure of information about amounts recognized in balance sheet [Line Items] |  |  |
Net deficit to be shown in the balance sheet | (16,199) | (14,504) |
Present value of defined benefit obligation [Member] | Unfunded [member] |  |  |
Disclosure of information about amounts recognized in balance sheet [Line Items] |  |  |
Net deficit to be shown in the balance sheet | (1,446) | (1,724) |
Present value of defined benefit obligation [Member] | Pension benefits [member] |  |  |
Disclosure of information about amounts recognized in balance sheet [Line Items] |  |  |
Net deficit to be shown in the balance sheet | (16,647) | (15,302) |
Present value of defined benefit obligation [Member] | Pension benefits [member] | Funded [member] |  |  |
Disclosure of information about amounts recognized in balance sheet [Line Items] |  |  |
Net deficit to be shown in the balance sheet | (16,199) |  |
Present value of defined benefit obligation [Member] | Pension benefits [member] | Unfunded [member] |  |  |
Disclosure of information about amounts recognized in balance sheet [Line Items] |  |  |
Net deficit to be shown in the balance sheet | (448) |  |
Present value of defined benefit obligation [Member] | Other Benefits [member] |  |  |
Disclosure of information about amounts recognized in balance sheet [Line Items] |  |  |
Net deficit to be shown in the balance sheet | (998) | (926) |
Present value of defined benefit obligation [Member] | Other Benefits [member] | Unfunded [member] |  |  |
Disclosure of information about amounts recognized in balance sheet [Line Items] |  |  |
Net deficit to be shown in the balance sheet | (998) |  |
Effect of asset ceiling [Member] |  |  |
Disclosure of information about amounts recognized in balance sheet [Line Items] |  |  |
Net deficit to be shown in the balance sheet | (111) | (63) | $ (49)
Effect of asset ceiling [Member] | Pension benefits [member] |  |  |
Disclosure of information about amounts recognized in balance sheet [Line Items] |  |  |
Net deficit to be shown in the balance sheet | $ (111) | $ (63) |

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- Definition The present value, without deducting any plan assets, of expected future payments required to settle the obligation resulting from employee service in the current and prior periods. [Refer: Plan assets, at fair value] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 19 -IssueDate 2016-01-13 -Paragraph 57 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_57_a&doctype=Standard -URIDate 2016-03-31 + Details
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- Definition The amount of deficit or surplus in a defined benefit plan, adjusted for any effect of limiting a net defined benefit asset to the asset ceiling. [Refer: Defined benefit plans [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2016-01-13 -Paragraph 140 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_140_a&doctype=Standard -URIDate 2016-03-31 + Details
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- Definition The present value of the defined benefit obligation, less the fair value of the plan assets (if any). [Refer: Plan assets [member]] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 19 -IssueDate 2016-01-13 -Paragraph 57 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_57_a&doctype=Standard -URIDate 2016-03-31 + Details
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- Definition Deficit in plan. + References No definition available. + Details
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