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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
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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]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2020-01-01 -Paragraph 145 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2020-en-r&anchor=para_145_a&doctype=Standard -URIDate 2020-01-01 + Details
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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 -Name IAS -Number 19 -IssueDate 2020-01-01 -Paragraph 145 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2020-en-r&anchor=para_145_a&doctype=Standard -URIDate 2020-01-01 + Details
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