Description of employers contribution to employee share purchase plan.

No definition available.

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- Definition

Number of Deferred Share Units That Can Be Granted as Percentage of Shares Acquired

+ References

No definition available.

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- Definition

Share Based Compensation Arrangement By Share Based Payment Award Grant Date Fair Value

+ References

No definition available.

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- Definition

Share-based Compensation Arrangement by Share-base Payment Award, Options, Vested and Nonvested Options Forfeited, Number of Shares

+ References

No definition available.

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- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Ownership Target, Period

+ References

No definition available.

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- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Awards other than Options Forfeited, Number of Shares

+ References

No definition available.

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- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Ratio Employer Match Per Dollar Contributed

+ References

No definition available.

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- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Trading Days Used to Calculate Average Share Price

+ References

No definition available.

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- Definition

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.F)
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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Amount of cost (reversal of cost) for employee benefits and share-based payment arrangement. Includes, but is not limited to, pension, other postretirement, postemployment and termination benefits.

+ References

No definition available.

+ Details

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- Definition

Unrecognized cost of unvested share-based compensation awards.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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- Definition

Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

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- Definition

This item represents the amount of cash contributions during the period made by the entity to the Employee Stock Ownership Plan (ESOP).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 40
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=65895303&loc=d3e17540-113929

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- Definition

Amount of cost (reversal of cost) for pension and other postretirement benefits.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(b)(4))
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-Name Accounting Standards Codification
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-Publisher FASB
-Name Accounting Standards Codification
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- Definition

Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
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- Definition

The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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- Definition

The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(2)
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- Definition

The highest percentage of annual salary that an employee is permitted to utilize with respect to the plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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- Definition

Number of non-option equity instruments exercised by participants.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iv)(2)
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- Definition

The number of shares purchased on the open market during the period for issuance to employees under the plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
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- Definition

Price of a single share of a number of saleable stocks of a company.

+ References

No definition available.

+ Details

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- Definition

Percentage of vesting of share-based compensation awards.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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- Definition

Amount of capitalized and expensed compensation cost from equity-based compensation arrangements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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- Definition

Fair value of options vested. Excludes equity instruments other than options, for example, but not limited to, share units, stock appreciation rights, restricted stock.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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- Definition

Value, before forfeitures, of stock or other type of equity granted of any equity-based compensation plan other than an employee stock ownership plan (ESOP).

+ References

No definition available.

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Stock-Based Compensation - Narrative (Detail) CAD / shares in Units, CAD in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017 CAD shares	Dec. 31, 2017 CAD CAD / shares shares	Dec. 31, 2016 CAD shares	Dec. 31, 2015 CAD shares
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Stock-based compensation expense (recovery)		CAD 35	CAD 51	CAD 66
Company's closing stock price (in CAD per share) \| CAD / shares		CAD 229.66
Performance Share Units [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Stock-based compensation expense (recovery)		CAD 30	CAD 29	55
Total unrecognized compensation related to share based compensation plans		CAD 20
Expected to be recognized over a weighted-average period		1 year 6 months
Units issued \| shares		134,991	147,157
Grant date fair value		CAD 27
Other than options vesting period		3 years
Payout Percentage		160.00%	118.00%
PSUs Vested in Period \| shares		82,673
PSU payout amount	CAD 31	CAD 30
Number of trading days		30 days	30 days
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Exercised \| shares	133,728	133,728	83,466
Deferred Share Units [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Stock-based compensation expense (recovery)		CAD (3)	CAD 2	(10)
Total unrecognized compensation related to share based compensation plans		CAD 1
Expected to be recognized over a weighted-average period		1 year 4 months 24 days
Units issued \| shares		23,932	31,069
Grant date fair value		CAD 5
Other than options vesting period		48 months
Matching no. of DSU's that may be granted to key employees to meet share ownership target when acquiring Common Shares and DSU's acquired		25.00%
Ownership target threshold		5 years
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Exercised \| shares		33,682	87,996
Regular and Performance Stock Options [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Stock-based compensation expense (recovery)		CAD 3	CAD 14	15
Stock Granted, Value, Share-based Compensation, Gross		17	16	18
Total unrecognized compensation related to share based compensation plans		CAD 9
Expected to be recognized over a weighted-average period		1 year 3 months 18 days
Total fair value of shares vested for stock option plan		CAD 14	15	17
Employee Share Purchase Plan [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Stock-based compensation expense (recovery)		CAD 4	CAD 5	CAD 4
Other than options vesting period		1 year
Employer contribution to employee share purchase plan		$1 for every $3 contributed by employees
The highest percentage of annual salary that an employee is permitted to utilize with respect to the plan		6.00%
Ratio employer matches per dollar contributed		0.3333
Total number of shares purchased on behalf of participants \| shares		130,041	140,560	131,703
Employer contributions		CAD 5	CAD 5	CAD 5
Chief Executive Officer [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Employee Benefits and Share-based Compensation		(51)
Pension and Other Postretirement Benefits Cost (Reversal of Cost)		CAD (27)
Chief Executive Officer [Member] \| Performance Share Units [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Forfeited awards other than Options, Number of Shares \| shares		22,514
Chief Executive Officer [Member] \| Deferred Share Units [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Forfeited awards other than Options, Number of Shares \| shares		68,612
Chief Executive Officer [Member] \| Regular and Performance Stock Options [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Share-based Compensation Arrangement by Share-base Payment Award, Options, Vested and Nonvested Options Forfeited, Number of Shares \| shares		752,145