Employee Future Benefits - Summary of Potential Impact on Obligations Arising From Changes in Key Assumptions (Detail) $ in Millions	Dec. 31, 2020 CAD ($)
Actuarial assumption of discount rates [Member] \| Pension defined benefit plans [member]
Disclosure of sensitivity analysis for actuarial assumptions [Line Items]
Impact of increase in actuarial assumption	$ (467)
Impact of decrease in actuarial assumption	554
Actuarial assumption of discount rates [Member] \| Retiree welfare plans [Member]
Disclosure of sensitivity analysis for actuarial assumptions [Line Items]
Impact of increase in actuarial assumption	(67)
Impact of decrease in actuarial assumption	82
Actuarial assumption of medical cost trend rates [Member] \| Retiree welfare plans [Member]
Disclosure of sensitivity analysis for actuarial assumptions [Line Items]
Impact of increase in actuarial assumption	20
Impact of decrease in actuarial assumption	(17)
Actuarial assumption of decrease in inflation rates [Member] \| Pension defined benefit plans [member]
Disclosure of sensitivity analysis for actuarial assumptions [Line Items]
Impact of decrease in actuarial assumption	143
Actuarial assumption of decrease in inflation rates [Member] \| Retiree welfare plans [Member]
Disclosure of sensitivity analysis for actuarial assumptions [Line Items]
Impact of decrease in actuarial assumption	$ 16

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 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
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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

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