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Post-retirement benefits - Summary of maturity of defined benefit obligations (Detail)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disclosure of defined benefit plans [Line Items]
Proportion relating to current employees	20.00%	19.00%	20.00%
Proportion relating to former employees not yet retired	12.00%	11.00%	11.00%
Proportion relating to retirees	68.00%	70.00%	69.00%
Defined benefit obligation	100.00%	100.00%	100.00%
Average duration of obligations (years)	14 years 4 months 24 days	13 years 4 months 24 days	13 years 10 months 24 days
Pension benefits [member]
Disclosure of defined benefit plans [Line Items]
Proportion relating to current employees	20.00%
Proportion relating to former employees not yet retired	12.00%
Proportion relating to retirees	68.00%
Defined benefit obligation	100.00%
Average duration of obligations (years)	14 years 6 months
Other Benefits [member]
Disclosure of defined benefit plans [Line Items]
Proportion relating to current employees	19.00%
Proportion relating to former employees not yet retired	0.00%
Proportion relating to retirees	81.00%
Defined benefit obligation	100.00%
Average duration of obligations (years)	13 years 4 months 24 days

X

- 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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X

- Definition

The weighted average duration of a defined benefit obligation. [Refer: Weighted average [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2019-01-01
-Paragraph 147
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2019-en-r&anchor=para_147_c&doctype=Standard
-URIDate 2019-03-27

+ Details

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X

- Definition

Defined benefit obligation maturity percentage.

+ References

No definition available.

+ Details

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- Definition

Defined benefit obligations relating to current employees maturity percentage.

+ References

No definition available.

+ Details

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- Definition

Defined benefit obligations relating to former employees not yet retired maturity percentage.

+ References

No definition available.

+ Details

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X

- Definition

Defined benefit obligations relating to retirees maturity percentage.

+ References

No definition available.

+ Details

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- Details

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- Details

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