Pensions and Other Post-Employment Benefits - Additional Information (Detail) $ in Millions		12 Months Ended
Dec. 31, 2020 Age	Dec. 31, 2020 CAD ($) Age
Bottom of Range [Member]
Disclosure Of Defined Benefit Plans [Line Items]
Defined benefit plan, equity securities assets allocation percentage	25.00%
Defined benefit plan, fixed income assets allocation percentage	25.00%
Defined benefit plan, real estate assets allocation percentage	0.00%
Defined benefit plan, listed infrastructure assets allocation percentage	0.00%
Defined benefit plan, emerging market debts assets allocation percentage	0.00%
Defined benefit plan, cash and cash equivalents assets allocation percentage	0.00%
Top of Range [Member]
Disclosure Of Defined Benefit Plans [Line Items]
Defined benefit plan, equity securities assets allocation percentage	70.00%
Defined benefit plan, fixed income assets allocation percentage	35.00%
Defined benefit plan, real estate assets allocation percentage	15.00%
Defined benefit plan, listed infrastructure assets allocation percentage	10.00%
Defined benefit plan, emerging market debts assets allocation percentage	10.00%
Defined benefit plan, cash and cash equivalents assets allocation percentage	10.00%
OPEB [Member]
Disclosure Of Defined Benefit Plans [Line Items]
Maximum age for retired employees eligible for health care dental and life insurance benefits \| Age	65	65
Weighted average duration		13 years 3 months 18 days
Pension Benefits [Member]
Disclosure Of Defined Benefit Plans [Line Items]
Weighted average duration		17 years 4 months 24 days
Percentage of employees contribution under pension plan		4.00%
Employer contribution \| $		$ 10

Defined benefit plan, cash and cash equivalents assets allocation percentage.

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- Definition

Defined benefit plan, emerging market debts assets allocation percentage.

+ References

No definition available.

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- Definition

Defined benefit plan employee minimum contribution percentage.

+ References

No definition available.

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- Definition

Defined benefit plan, equity instruments assets allocation percentage.

+ References

No definition available.

+ Details

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- Definition

Defined benefit plan, fixed income assets allocation percentage.

+ References

No definition available.

+ Details

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- Definition

Defined benefit plan, listed infrastructure assets allocation percentage.

+ References

No definition available.

+ Details

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- Definition

Defined benefit plan, real estate assets allocation percentage.

+ References

No definition available.

+ Details

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- Definition

Maximum age for retired employees eligible for health care dental and life insurance benefits.

+ References

No definition available.

+ Details

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- Definition

Weighted average duration of defined benefit obligations.

+ References

No definition available.

+ Details

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- Definition

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.

+ References

No definition available.

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- Definition

The amount of employer contributions to retirement benefit plans.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 26
-IssueDate 2020-01-01
-Paragraph 35
-Subparagraph b
-Clause i
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=26&code=ifrs-tx-2020-en-r&anchor=para_35_b_i&doctype=Standard
-URIDate 2020-01-01

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