Pensions and Other Benefits - Additional Information (Detail) - CAD CAD in Millions	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Canadian pension plan as a percentage of combined plan assets	99.00%
Canadian pension plan as a percentage of combined plan obligations	98.00%
Percentage of combined plan obligations	96.00%
Period of average market values for calculating net periodic benefit cost	5 years
Increase (decrease) in pension and post-retirement benefit obligation			CAD 225
Target long term rate of return, net of all fees and expenses	7.75%	7.75%	7.75%
Maximum value of underlying assets represented by financial derivatives, excluding currency forwards	30.00%
Company's common shares in fund assets, Shares	109,630	188,276
Contribution by the company	CAD 57	CAD 90	CAD 88
Payments to employees, their beneficiaries or estates or to third-party benefit administrators	30	33	26
Net cost of defined contribution plan	9	9	8
Multi-employer plan contributions	4	4	4
Equity Securities [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Company's securities in fund assets	21	33
Debt Securities [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Company's securities in fund assets	3	3
Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Defined benefit pension plans' accumulated benefit obligation	11,143	10,893
Unamortized actuarial loss expected to be recognized in next fiscal year	153
Unamortized prior service cost expected to be recognized in next fiscal year	5
Contribution by the company	48	81
Other Postretirement Benefit Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Unamortized actuarial loss expected to be recognized in next fiscal year	2
Unamortized prior service cost expected to be recognized in next fiscal year	1
Contribution by the company	30	33
Other Pension Plans, Postretirement or Supplemental Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Contribution by the company	CAD 12	12	13
US Dollar [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Plans exposure to foreign currency risk percentage	34.00%
European Currencies [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Plans exposure to foreign currency risk percentage	14.00%
Other Currency [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Plans exposure to foreign currency risk percentage	5.00%
Canadian Registered and U.S. Qualified Defined Benefit Pension Plans [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Contribution by the company	CAD 36	69	67
Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Contribution by the company	CAD 9	CAD 9	CAD 8

Defined Benefit Plan, Benefit Obligation, Mortality Table Changes, Period Increase (Decrease)

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Domestic Pension Plan as Percentage of Combined Pension Plan Assets

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Expected Return On Fund Assets Calculated Using Market Related Assets Values Developed, Period

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Maximum value of underlying asset represented by financial derivatives, excluding currency forwards.

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Payments to employees, their beneficiaries or estates or to third-party benefit administrators.

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Percentage of combined plan obligations, other benefit plans.

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Percentage of defined benefit pension plan assets exposed to foreign currency risk.

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For defined benefit pension plans, the actuarial present value of benefits (whether vested or nonvested) attributed by the pension benefit formula to employee service rendered before a specified date and based on employee service and compensation (if applicable) before that date. The accumulated benefit obligation differs from the projected benefit obligation in that it includes no assumption about future compensation levels. For plans with flat-benefit or nonpay-related pension benefit formulas, the accumulated benefit obligation and the projected benefit obligation are the same.

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The amount of employer securities or securities of parties related to the employer that are included in plan assets.

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An assumption as to the rate of return on plan assets reflecting the average rate of earnings expected on the funds invested or to be invested to provide for the benefits included in the benefit obligation.

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Amount of contributions made by the employer to defined benefit plans.

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Amount included in accumulated other comprehensive income (AOCI) related to gain (loss) expected to be recognized as components of net periodic benefit cost for the fiscal year that follows the most recent annual statement of financial position.

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Amount included in accumulated other comprehensive income (AOCI) related to prior service cost (credit) expected to be recognized as components of net periodic benefit cost for the fiscal year that follows the most recent annual statement of financial position.

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The number of employer shares or related party shares included in plan assets.

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The amount of the cost recognized during the period for defined contribution plans.

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Amount of discretionary contributions made by an employer to a defined contribution plan.

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Amount of contributions made to multiemployer plans by the employer. A multiemployer plan is a pension or postretirement benefit plan to which two or more unrelated employers contribute where assets contributed by one participating employer may be used to provide benefits to employees of other participating employers.

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