employee future benefits - Plan investment strategies and policies (Details)	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Pension plans
employee future benefits
Present value-weighted average timing of estimate cash flows for obligations (in years)	14 years 1 month 6 days	13 years 8 months 12 days
Percentage of plan assets at end of year	100.00%	100.00%
Pension plans \| Equity
employee future benefits
Percentage of plan assets at end of year	38.00%	36.00%
Pension plans \| Equity \| Minimum
employee future benefits
Target future allocation	25.00%
Pension plans \| Equity \| Maximum
employee future benefits
Target future allocation	55.00%
Pension plans \| Equity \| Foreign \| Minimum
employee future benefits
Target future allocation	20.00%
Pension plans \| Equity \| Foreign \| Maximum
employee future benefits
Target future allocation	30.00%
Pension plans \| Debt securities
employee future benefits
Percentage of plan assets at end of year	51.00%	53.00%
Pension plans \| Debt securities \| Minimum
employee future benefits
Target future allocation	40.00%
Pension plans \| Debt securities \| Maximum
employee future benefits
Target future allocation	75.00%
Pension plans \| Real estate
employee future benefits
Percentage of plan assets at end of year	11.00%	11.00%
Pension plans \| Real estate \| Minimum
employee future benefits
Target future allocation	10.00%
Pension plans \| Real estate \| Maximum
employee future benefits
Target future allocation	30.00%
Pension plans \| Other assets \| Minimum
employee future benefits
Target future allocation	0.00%
Pension plans \| Other assets \| Maximum
employee future benefits
Target future allocation	15.00%
Other defined benefit plans
employee future benefits
Present value-weighted average timing of estimate cash flows for obligations (in years)	5 years 9 months 18 days	6 years 2 months 12 days

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.

No definition available.

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- 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 2020-01-01
-Paragraph 147
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2020-en-r&anchor=para_147_c&doctype=Standard
-URIDate 2020-01-01

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Target future allocation of investments of this type to total plan assets presented on a weighted average basis.

+ References

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- Definition

Percentage of fair value of investments (categorized by debt securities, equity securities, real estate and other plan assets) to the fair value of plan assets held.

+ References

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