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Post-retirement benefits - Summary of funding policy and contributions to plans (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure of net defined benefit liability (asset) [Line Items]
Contributions to defined benefit plans	$ 404	$ 464	$ 328
Contributions to defined contribution plans	243	240	272
Contributions to industry-wide plans	12	17	17
Total	659	$ 721	$ 617
Pension benefits [member]
Disclosure of net defined benefit liability (asset) [Line Items]
Contributions to defined benefit plans	349
Contributions to defined contribution plans	241
Contributions to industry-wide plans	12
Total	602
Other Benefits [member]
Disclosure of net defined benefit liability (asset) [Line Items]
Contributions to defined benefit plans	55
Contributions to defined contribution plans	2
Total	$ 57

X

- Definition

The decrease (increase) in net defined benefit liability (asset) resulting from contributions to a defined benefit plan by the employer. [Refer: Net defined benefit liability (asset); Defined benefit plans [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2016-01-13
-Paragraph 141
-Subparagraph f
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_141_f&doctype=Standard
-URIDate 2016-03-31

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X

- Definition

The decrease (increase) in the net defined benefit liability (asset) resulting from contributions to a defined benefit plan. [Refer: Net defined benefit liability (asset); Defined benefit plans [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2016-01-13
-Paragraph 141
-Subparagraph f
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_141_f&doctype=Standard
-URIDate 2016-03-31

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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.

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- Definition

The amount of employer contributions to retirement benefit plans.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 26
-IssueDate 2016-01-13
-Paragraph 35
-Subparagraph b
-Clause i
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=26&date=2016-03-30&anchor=para_35_b_i&doctype=Standard
-URIDate 2016-03-31

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- Definition

The decrease (increase) in the net defined benefit liability (asset) resulting from employer contributions to industry wide plans.

+ References

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