Pensions and Other Post Retirement Benefits - Actuarial assumptions (Details) - CAD ($) $ in Millions	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022
Pensions and Other Post Retirement Benefits
Actual experience period	5 years
Pension Benefits
Pensions and Other Post Retirement Benefits
Discount rate	4.60%	5.10%
Rate of compensation increase	3.00%	3.00%
Pension Benefits \| Discount rate
Pensions and Other Post Retirement Benefits
Increase in actuarial assumption (as a percent)	1.00%
Increase (decrease) in interest and service cost due to increase in actuarial assumption	$ (17)
Increase (decrease) in service and interest cost due to decrease in actuarial assumption	20
Increase (decrease) in net obligation due to increase in actuarial assumption	(767)
Increase (decrease) in net obligation due to decrease in actuarial assumption	$ 963
Other Post-Retirement Benefits
Pensions and Other Post Retirement Benefits
Discount rate	4.60%	5.10%
Rate of compensation increase	3.00%	3.00%
Assumption of annual percentage of increase of healthcare costs	5.00%
Other Post-Retirement Benefits \| Discount rate
Pensions and Other Post Retirement Benefits
Increase in actuarial assumption (as a percent)	1.00%
Increase (decrease) in net obligation due to increase in actuarial assumption	$ (58)
Increase (decrease) in net obligation due to decrease in actuarial assumption	$ 70
Other Post-Retirement Benefits \| Health care cost
Pensions and Other Post Retirement Benefits
Increase in actuarial assumption (as a percent)	1.00%
Increase (decrease) in interest and service cost due to increase in actuarial assumption	$ 1
Increase (decrease) in service and interest cost due to decrease in actuarial assumption	(1)
Increase (decrease) in net obligation due to increase in actuarial assumption	22
Increase (decrease) in net obligation due to decrease in actuarial assumption	$ (19)

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]]

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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The expected rate of increases in pension that is 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]]

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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]]

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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
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-Number 19
-IssueDate 2023-01-01
-Paragraph 145
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-URIDate 2023-03-23

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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]]

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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]]

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Actual experience period.

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Increase (decrease) in service and interest cost due to decrease in actuarial assumption.

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The increase (decrease) in interest and service cost that would have been caused by an increase in a significant actuarial assumption that was reasonably possible at the end of the reporting period.

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