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Post-retirement benefits - Summary of main assumptions rate for valuations of plans (Parenthetical) (Detail) | Dec. 31, 2017 | Dec. 31, 2016
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Disclosure Of Net Defined Benefit Liability Asset [Abstract] |  |
Assumption for the consumer price index | 2.10% | 2.20%

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- Definition Any other material assumption used as a significant actuarial assumption to determine the present value of a defined benefit obligation. [Refer: Actuarial assumptions [member]; Defined benefit obligation, at present value] + 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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