Employee Future Benefits - Summary of Pension and Retiree Welfare Plans (Detail) - CAD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Actuarial losses (gains) due to:
Ending balance	$ 0
Pension defined benefit plans [member]
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	368
Actuarial losses (gains) due to:
Experience	2	$ 8
Demographic assumption changes	(67)
Economic assumption changes	333	413
Return on plan assets (excluding interest income)	(310)	(529)
Ending balance	306	368
Pension defined benefit plans [member] \| Present value of defined benefit obligation [Member]
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	4,817	4,675
Current service cost	41	40
Interest cost	150	182
Plan participants' contributions		1
Actuarial losses (gains) due to:
Experience	2	8
Demographic assumption changes	(67)
Economic assumption changes	333	413
Benefits paid	(318)	(358)
Impact of changes in foreign exchange rates	(57)	(144)
Ending balance	4,901	4,817
Pension defined benefit plans [member] \| Plan assets [Member]
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	4,453	4,187
Interest cost	140	164
Plan participants' contributions		1
Actuarial losses (gains) due to:
Benefits paid	(318)	(358)
Impact of changes in foreign exchange rates	(54)	(136)
Return on plan assets (excluding interest income)	310	529
Ending balance	4,595	4,453
Employer contributions	71	75
Administration costs	(7)	(9)
Retiree welfare plans [Member]
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	47
Actuarial losses (gains) due to:
Experience	(14)	(10)
Demographic assumption changes	(12)
Economic assumption changes	49	56
Return on plan assets (excluding interest income)	(33)	(25)
Ending balance	32	47
Retiree welfare plans [Member] \| Present value of defined benefit obligation [Member]
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	645	640
Interest cost	20	25
Plan participants' contributions	3	3
Actuarial losses (gains) due to:
Experience	(14)	(10)
Demographic assumption changes	(12)
Economic assumption changes	49	56
Benefits paid	(45)	(46)
Impact of changes in foreign exchange rates	(8)	(23)
Ending balance	638	645
Retiree welfare plans [Member] \| Plan assets [Member]
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	598	610
Interest cost	19	25
Plan participants' contributions	3	3
Actuarial losses (gains) due to:
Benefits paid	(45)	(46)
Impact of changes in foreign exchange rates	(11)	(29)
Return on plan assets (excluding interest income)	33	25
Ending balance	606	598
Employer contributions	11	12
Administration costs	$ (2)	$ (2)

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 2020-01-01
-Paragraph 141
-Subparagraph c
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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

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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
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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
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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.

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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 2020-01-01
-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
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-Paragraph 141
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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 2020-01-01
-Paragraph 140
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The decrease (increase) in the net defined benefit liability (asset) resulting from payments from the plan. [Refer: Net defined benefit liability (asset)]

+ References

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-Name IAS
-Number 19
-IssueDate 2020-01-01
-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 plans 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]; Interest expense (income), net defined benefit liability (asset)]

+ References

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- Definition

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

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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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