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Pensions and Other Post-Employment Benefits - Sensitivity of Defined Benefit and OPEB Obligation to Changes in Relevant Actuarial Assumptions (Detail) - CAD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Discount Rate [Member]
Disclosure Of Sensitivity Analysis For Actuarial Assumptions [Line Items]
Increase	$ (31)	$ (25)
Decrease	$ 40	$ 32
Sensitivity Range - One percent change	1.00%	1.00%
Future Salary Growth Rate [Member]
Disclosure Of Sensitivity Analysis For Actuarial Assumptions [Line Items]
Increase	$ 4	$ 3
Decrease	$ (4)	$ (3)
Sensitivity Range - One percent change	1.00%	1.00%
Health Care Cost Trend Rate [Member]
Disclosure Of Sensitivity Analysis For Actuarial Assumptions [Line Items]
Increase	$ 1	$ 1
Decrease	$ (1)	$ (1)
Sensitivity Range - One percent change	1.00%	1.00%
One Year Change in Assumed Life Expectancy [Member]
Disclosure Of Sensitivity Analysis For Actuarial Assumptions [Line Items]
Increase	$ 4	$ 3
Decrease	$ (4)	$ (3)
Sensitivity Range - One percent change	1.00%	1.00%

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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]]

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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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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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