Employee Future Benefits - Summary of Pension and Retiree Welfare Plans (Detail) - CAD ($) $ in Millions	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020
Pension defined benefit plans [member]
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	$ 306
Actuarial losses (gains) due to:
Experience	3	$ 2
Demographic assumption changes	7	(67)
Economic assumption changes	(194)	333
Return on plan assets (excluding interest income)	(70)	(310)
Ending balance	87	306
Pension defined benefit plans [member] \| Present value of defined benefit obligation [Member]
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	4,901	4,817
Current service cost	44	41
Interest cost	115	150
Actuarial losses (gains) due to:
Experience	3	2
Demographic assumption changes	7	(67)
Economic assumption changes	(194)	333
Benefits paid	(303)	(318)
Impact of changes in foreign exchange rates	(13)	(57)
Ending balance	4,560	4,901
Pension defined benefit plans [member] \| Plan assets [Member]
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	4,595	4,453
Interest cost	109	140
Actuarial losses (gains) due to:
Benefits paid	(303)	(318)
Impact of changes in foreign exchange rates	(13)	(54)
Return on plan assets (excluding interest income)	70	310
Employer contributions	61	71
Administration costs	(9)	(7)
Ending balance	4,510	4,595
Retiree welfare plans [Member]
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	32
Actuarial losses (gains) due to:
Experience		(14)
Demographic assumption changes	1	(12)
Economic assumption changes	(29)	49
Return on plan assets (excluding interest income)	1	(33)
Ending balance	(3)	32
Retiree welfare plans [Member] \| Present value of defined benefit obligation [Member]
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	638	645
Interest cost	15	20
Plan participants' contributions	3	3
Actuarial losses (gains) due to:
Experience		(14)
Demographic assumption changes	1	(12)
Economic assumption changes	(29)	49
Benefits paid	(42)	(45)
Impact of changes in foreign exchange rates	(2)	(8)
Ending balance	584	638
Retiree welfare plans [Member] \| Plan assets [Member]
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	606	598
Interest cost	14	19
Plan participants' contributions	3	3
Actuarial losses (gains) due to:
Benefits paid	(42)	(45)
Impact of changes in foreign exchange rates	(2)	(11)
Return on plan assets (excluding interest income)	(1)	33
Employer contributions	11	11
Administration costs	(2)	(2)
Ending balance	$ 587	$ 606

The decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from changes in demographic assumptions that result in remeasurements of the net defined benefit liability (asset). Demographic assumptions deal with matters such as: (a) mortality; (b) rates of employee turnover, disability and early retirement; (c) the proportion of plan members with dependants who will be eligible for benefits; (d) the proportion of plan members who will select each form of payment option available under the plan terms; and (e) claim rates under medical plans. [Refer: Net defined benefit liability (asset)]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2021-01-01
-Paragraph 141
-Subparagraph c
-Clause ii
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2021-en-r&anchor=para_141_c_ii&doctype=Standard
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- Definition

The decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from changes in financial assumptions that result in remeasurements of the net defined benefit liability (asset). Financial assumptions deal with items such as: (a) the discount rate; (b) benefit levels, excluding any cost of the benefits to be met by employees, and future salary; (c) in the case of medical benefits, future medical costs, including claim handling costs (ie the costs that will be incurred in processing and resolving claims, including legal and adjuster's fees); and (d) taxes payable by the plan on contributions relating to service before the reporting date or on benefits resulting from that service. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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-Number 19
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- Definition

The decrease (increase) in net defined benefit liability (asset) resulting from contributions to a defined benefit plan by the employer. [Refer: Net defined benefit liability (asset); Defined benefit plans [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
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- Definition

The decrease (increase) in the net defined benefit liability (asset) resulting from contributions to a defined benefit plan by plan participants. [Refer: Net defined benefit liability (asset); Defined benefit plans [member]]

+ References

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- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from employee service in the current period. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2021-01-01
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Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with members defined in one or many axes of the table.

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No definition available.

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- References

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- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from changes in foreign exchange rates. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2021-01-01
-Paragraph 141
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- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from the passage of time. [Refer: Interest expense; Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2021-01-01
-Paragraph 141
-Subparagraph b
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- Definition

The amount of deficit or surplus in a defined benefit plan, adjusted for any effect of limiting a net defined benefit asset to the asset ceiling. [Refer: Defined benefit plans [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2021-01-01
-Paragraph 140
-Subparagraph a
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- Definition

The decrease (increase) in the net defined benefit liability (asset) resulting from payments from the plan. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2021-01-01
-Paragraph 141
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- Definition

The decrease (increase) in the net defined benefit liability (asset) resulting from the return on plan assets, excluding amounts included in interest income or expense. The return on plan assets is interest, dividends and other revenue derived from the plan assets, together with realised and unrealised gains or losses on the plan assets, less any costs of managing plan assets and less any tax payable by the plan itself, other than tax included in the actuarial assumptions used to measure the present value of the defined benefit obligation. [Refer: Plan assets [member]; Net defined benefit liability (asset); Actuarial assumptions [member]; Increase (decrease) in net defined benefit liability (asset) resulting from interest expense (income)]

+ References

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-Name IAS
-Number 19
-IssueDate 2021-01-01
-Paragraph 141
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- Definition

Actuarial gains (losses) arising from experience, net defined benefit liability (asset).

+ References

No definition available.

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- Definition

Administration costs, net defined benefit liability (asset).

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No definition available.

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