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Pension and other post-retirement benefits - Assumptions and 1% change (Details) - CAD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Pension plans [member]
Assumptions defined benefit obligation and net pension and other benefit expense
Discount rate - obligation	2.40%	3.00%
Discount rate - expense	3.00%	3.70%
Rate of compensation increase	2.90%	3.00%
Pension plans [member] | Discount rate [member]
Sensitivity analysis of actuarial assumptions
Increase in actuarial assumption by	1.00%
Increase in defined benefit obligation	$ 12,766,000
Decrease in actuarial assumption by	1.00%
Decrease in defined benefit obligation	$ (9,778,000)
Pension plans [member] | Rate of compensation increase [member]
Sensitivity analysis of actuarial assumptions
Increase in actuarial assumption by	1.00%
Increase in defined benefit obligation	$ 3,009,000
Decrease in actuarial assumption by	1.00%
Decrease in defined benefit obligation	$ (2,772,000)
Pension plans [member] | Increase of one year in the expected lifetime [member]
Sensitivity analysis of actuarial assumptions
Increase in defined benefit obligation	$ 2,030,000
Other Benefit Plans [Member]
Assumptions defined benefit obligation and net pension and other benefit expense
Discount rate - obligation	2.50%	3.10%
Discount rate - expense	3.10%	3.90%
Initial health care cost trend rate	5.00%	6.00%
Cost trend rate declines to	5.00%	5.00%
Year the rate reaches its final level	2021	2022
Dental care cost trend rate	4.50%	5.00%
Other Benefit Plans [Member] | Discount rate [member]
Sensitivity analysis of actuarial assumptions
Increase in actuarial assumption by	1.00%
Increase in defined benefit obligation	$ 4,138,000
Decrease in actuarial assumption by	1.00%
Decrease in defined benefit obligation	$ (3,270,000)

X

- Definition

Declining cost rate related to medical costs used as significant actuarial assumption to determine the present value of a defined benefit obligation.

+ References

No definition available.

+ Details

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X

- Definition

Dental cost trend rate used as significant actuarial assumption to determine the present value of a defined benefit obligation.

+ References

No definition available.

+ Details

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X

- Definition

The discount rate used as the significant actuarial assumption to determine the present value of a defined benefit expense.

+ References

No definition available.

+ Details

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- Definition

The year in which the rate reaches its final level, in CCYY format.

+ References

No definition available.

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- References

No definition available.

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- Definition

The discount rate used as the significant actuarial assumption to determine the present value of a defined benefit obligation. [Refer: Defined benefit obligation, at present value; Actuarial assumptions [member]]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 19
-IssueDate 2020-01-01
-Paragraph 144
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X

- Definition

The expected rate of salary increases used as a significant actuarial assumption to determine the present value of a defined benefit obligation. [Refer: Defined benefit plans [member]; Defined benefit obligation, at present value; Actuarial assumptions [member]]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 19
-IssueDate 2020-01-01
-Paragraph 144
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2020-en-r&anchor=para_144&doctype=Standard
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- Definition

Medical cost trend rate used as significant actuarial assumption to determine the present value of a defined benefit obligation. [Refer: Defined benefit obligation, at present value; Actuarial assumptions [member]]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
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-IssueDate 2020-01-01
-Paragraph 144
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- References

No definition available.

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- Definition

The increase (decrease) in a defined benefit obligation that would have been caused by a decrease in a significant actuarial assumption that was reasonably possible at the end of the reporting period. [Refer: Actuarial assumptions [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2020-01-01
-Paragraph 145
-Subparagraph a
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- Definition

The increase (decrease) in a defined benefit obligation that would have been caused by an increase in a significant actuarial assumption that was reasonably possible at the end of the reporting period. [Refer: Actuarial assumptions [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition

The reasonably possible percentage of the decrease in the actuarial assumption used to determine the present value of defined benefit obligation. [Refer: Actuarial assumptions [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2020-01-01
-Paragraph 145
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- Definition

The reasonably possible percentage of the increase in the actuarial assumption used to determine the present value of defined benefit obligation. [Refer: Actuarial assumptions [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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-IssueDate 2020-01-01
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