Employee Future Benefits - Summary of Amounts Recognized in Consolidated Statements of Financial Position (Detail) - CAD ($)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Disclosure of defined benefit plans [Line Items]
Deficit and net defined benefit liability		$ 0
Pension defined benefit plans [member]
Disclosure of defined benefit plans [Line Items]
Defined benefit obligation	$ 4,675,000,000	4,706,000,000
Fair value of plan assets	4,190,000,000	4,328,000,000
Deficit	485,000,000	378,000,000
Effect of asset limit	9,000,000
Deficit and net defined benefit liability	494,000,000	378,000,000
Pension defined benefit plans [member] \| Funded [Member]
Disclosure of defined benefit plans [Line Items]
Deficit and net defined benefit liability	(248,000,000)	(383,000,000)
Pension defined benefit plans [member] \| Unfunded [Member]
Disclosure of defined benefit plans [Line Items]
Deficit and net defined benefit liability	742,000,000	761,000,000
Retiree welfare plans [Member]
Disclosure of defined benefit plans [Line Items]
Defined benefit obligation	640,000,000	665,000,000
Fair value of plan assets	610,000,000	587,000,000
Deficit	30,000,000	78,000,000
Deficit and net defined benefit liability	30,000,000	78,000,000
Retiree welfare plans [Member] \| Funded [Member]
Disclosure of defined benefit plans [Line Items]
Deficit and net defined benefit liability	(121,000,000)	(72,000,000)
Retiree welfare plans [Member] \| Unfunded [Member]
Disclosure of defined benefit plans [Line Items]
Deficit and net defined benefit liability	$ 151,000,000	$ 150,000,000

The present value, without deducting any plan assets, of expected future payments required to settle the obligation resulting from employee service in the current and prior periods. [Refer: Plan assets, at fair value]

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 19
-IssueDate 2018-01-01
-Paragraph 57
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2018-03-01&anchor=para_57_a&doctype=Standard
-URIDate 2018-03-16

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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 decrease (increase) in the net defined benefit liability (asset) resulting from the gain (loss) on changes in the effect of limiting a net defined benefit asset to the asset ceiling, excluding amounts included in interest income or expense. [Refer: Interest income; Net defined benefit liability (asset); Interest expense (income), net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2018-01-01
-Paragraph 141
-Subparagraph c
-Clause iv
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2018-03-01&anchor=para_141_c_iv&doctype=Standard
-URIDate 2018-03-16

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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 2018-01-01
-Paragraph 140
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2018-03-01&anchor=para_140_a&doctype=Standard
-URIDate 2018-03-16

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- Definition

The fair value of defined benefit plan assets. Plan assets comprise assets held by a long-term employee benefit fund and qualifying insurance policies. [Refer: At fair value [member]]

+ References

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- Definition

The fair value of any plan assets, less the present value of the defined benefit obligation. [Refer: Plan assets [member]]

+ References

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