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Post-retirement benefits - Summary of amount recognised in balance sheet (Detail) - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
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Disclosure of information about amounts recognized in balance sheet [Line Items] |  |  |
Net deficit to be shown in the balance sheet | $ (1,551) | $ (2,499) | $ (2,542)
Effect of asset ceiling |  | (111) |
Deficits | (2,486) | (3,370) |
Surpluses | 935 | 871 |
Net deficits on pension plans | (680) | (1,501) |
Unfunded [member] |  |  |
Disclosure of information about amounts recognized in balance sheet [Line Items] |  |  |
Unfunded post-retirement healthcare obligation | (871) | (998) |
Pension benefits [member] |  |  |
Disclosure of information about amounts recognized in balance sheet [Line Items] |  |  |
Net deficit to be shown in the balance sheet | (680) | (1,501) |
Deficits | (1,615) |  |
Surpluses | 935 |  |
Net deficits on pension plans | (680) |  |
Other Benefits [member] |  |  |
Disclosure of information about amounts recognized in balance sheet [Line Items] |  |  |
Net deficit to be shown in the balance sheet | (871) | (998) |
Deficits | (871) |  |
Other Benefits [member] | Unfunded [member] |  |  |
Disclosure of information about amounts recognized in balance sheet [Line Items] |  |  |
Unfunded post-retirement healthcare obligation | (871) |  |
Plan assets [Member] |  |  |
Disclosure of information about amounts recognized in balance sheet [Line Items] |  |  |
Net deficit to be shown in the balance sheet | 13,203 | 15,257 | 13,749
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 | 13,203 | 15,257 |
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 | (14,754) | (17,645) | $ (16,228)
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 | (13,482) | (16,199) |
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 | (1,272) | (1,446) |
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 | (13,883) | (16,647) |
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 | (13,482) |  |
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 | (401) |  |
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 | (871) | $ (998) |
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 | $ (871) |  |

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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 2017-01-01 -Paragraph 57 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_57_a&doctype=Standard -URIDate 2017-03-09 + 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 2017-01-01 -Paragraph 140 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_140_a&doctype=Standard -URIDate 2017-03-09 + 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 2017-01-01 -Paragraph 57 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_57_a&doctype=Standard -URIDate 2017-03-09 + Details
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- Definition Deficit in plan. + References No definition available. + Details
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- Definition Disclosure of information about amounts recognized in balance sheet [Line Items] + References No definition available. + Details
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- Definition The combined total of non-current and current pension surpluses classified as trade and other receivables. + References No definition available. + Details
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