Employee Future Benefits - Summary of Amounts Recognized in Consolidated Statements of Financial Position (Detail) - CAD	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Disclosure of defined benefit plans [Line Items]
Deficit and net defined benefit liability	CAD 0
Pension plans [Member]
Disclosure of defined benefit plans [Line Items]
Defined benefit obligation	4,706,000,000	CAD 4,767,000,000
Fair value of plan assets	4,328,000,000	4,277,000,000
Deficit	378,000,000	490,000,000
Effect of asset limit	0	0
Deficit and net defined benefit liability	378,000,000	490,000,000
Pension plans [Member] \| Funded [Member]
Disclosure of defined benefit plans [Line Items]
Deficit and net defined benefit liability	(383,000,000)	(292,000,000)
Pension plans [Member] \| Unfunded [Member]
Disclosure of defined benefit plans [Line Items]
Deficit and net defined benefit liability	761,000,000	782,000,000
Retiree welfare plans [Member]
Disclosure of defined benefit plans [Line Items]
Defined benefit obligation	665,000,000	682,000,000
Fair value of plan assets	587,000,000	603,000,000
Deficit	78,000,000	79,000,000
Effect of asset limit	0	0
Deficit and net defined benefit liability	78,000,000	79,000,000
Retiree welfare plans [Member] \| Funded [Member]
Disclosure of defined benefit plans [Line Items]
Deficit and net defined benefit liability	(72,000,000)	(63,000,000)
Retiree welfare plans [Member] \| Unfunded [Member]
Disclosure of defined benefit plans [Line Items]
Deficit and net defined benefit liability	CAD 150,000,000	CAD 142,000,000

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 2017-01-01
-Paragraph 57
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_57_a&doctype=Standard
-URIDate 2017-03-09

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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)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2017-01-01
-Paragraph 141
-Subparagraph c
-Clause iv
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_141_c_iv&doctype=Standard
-URIDate 2017-03-09

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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 2017-01-01
-Paragraph 140
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_140_a&doctype=Standard
-URIDate 2017-03-09

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

+ References

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