Pensions and Other Benefits - Information About DB Pension Plans and Other Benefits (Detail) - CAD CAD in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in fund assets:
Fair value of fund assets, Beginning	CAD 12,196
Fair value of fund assets, Ending	12,808	CAD 12,196
Pension Plans, Defined Benefit [Member]
Change in projected benefit obligation:
Benefit obligation at January 1	11,399	11,194
Current service cost	103	106	CAD 126
Interest cost	451	467	463
Employee contributions	44	40
Benefits paid	(648)	(645)
Foreign currency changes	(15)	(7)
Plan amendments and other	1	6
Actuarial loss (gain)	344	238
Projected benefit obligation at December 31	11,679	11,399	11,194
Change in fund assets:
Fair value of fund assets, Beginning	12,196	12,300
Actual return on fund assets	1,183	461
Employer contributions	46	48
Employee contributions	44	40
Benefits paid	(648)	(645)
Foreign currency changes	(13)	(8)
Fair value of fund assets, Ending	12,808	12,196	12,300
Funded status – plan surplus (deficit)	1,129	797
Other Postretirement Benefit Plans, Defined Benefit [Member]
Change in projected benefit obligation:
Benefit obligation at January 1	510	513
Current service cost	12	11	12
Interest cost	20	21	21
Employee contributions	1	1
Benefits paid	(35)	(31)
Foreign currency changes	(3)	0
Plan amendments and other	0	0
Actuarial loss (gain)	13	(5)
Projected benefit obligation at December 31	518	510	513
Change in fund assets:
Fair value of fund assets, Beginning	5	6
Actual return on fund assets	(1)	(1)
Employer contributions	34	30
Employee contributions	1	1
Benefits paid	(35)	(31)
Foreign currency changes	0	0
Fair value of fund assets, Ending	4	5	CAD 6
Pension plans in deficit [Member] \| Pension Plans, Defined Benefit [Member]
Change in projected benefit obligation:
Benefit obligation at January 1	497
Projected benefit obligation at December 31	505	497
Change in fund assets:
Fair value of fund assets, Beginning	224
Fair value of fund assets, Ending	227	224
Funded status – plan surplus (deficit)	(278)	(273)
Pension plans in deficit [Member] \| Other Postretirement Benefit Plans, Defined Benefit [Member]
Change in fund assets:
Funded status – plan surplus (deficit)	CAD (514)	CAD (505)

Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses.

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Amount of gain (loss) from change in actuarial assumptions which (increases) decreases benefit obligation of defined benefit plan. Assumptions include, but are not limited to, interest, mortality, employee turnover, salary, and temporary deviation from substantive plan.

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Amount of actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

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Amount of payment to participant of defined benefit plan which decreases benefit obligation. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount of contributions received by defined benefit plan from participant which increase benefit obligation.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount of contribution received by defined benefit plan from employer which increases plan assets.

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Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees.

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Amount of foreign currency translation gain (loss) which (increases) decreases benefit obligation of defined benefit plan.

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Amount of funded (unfunded) status of defined benefit plan, measured as difference between fair value of plan assets and benefit obligation. Includes, but is not limited to, overfunded (underfunded) status.

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Amount of cost recognized for passage of time related to defined benefit plan.

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Amount of increase (decrease) in benefit obligation of defined benefit plan from change in terms of existing plan or initiation of new plan.

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Amount of payment to participant under defined benefit plan which decreases plan assets. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services.

+ References

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Amount of contributions received by defined benefit plan from participant which increases plan assets.

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Amount of foreign currency translation gain (loss) which increases (decreases) plan assets of defined benefit plan.

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Amount of cost for actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

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