Stock-based Compensation (PSU Table) (Details) - Share Units Plan [Member] - CAD CAD / shares in Units, shares in Millions, CAD in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Equity Settled Awards [Member] \| PSU-ROIC [Member]
Roll forward of outstanding PSUs
Units outstanding at beginning of period	1.3
Units granted	0.4
Units settled	(0.4)
Units forfeited	(0.1)
Units outstanding at end of period	1.2	1.3
Weighted-average grant date fair value at beginning of period	CAD 49.82
Weighted-average grant date fair value of units granted in period	53.19
Weighted-average grant date fair value of units settled in period	66.84
Weighted-average grant date fair value of units forfeited in period	37.62
Weighted-average grant date fair value at end of period	CAD 46.35	CAD 49.82
Roll forward of nonvested PSUs
Units nonvested at beginning of period	0.9
Units granted in period	0.4
Units vested during the year	(0.4)
Units forfeited in period	(0.1)
Units nonvested at end of period	0.8	0.9
Weighted-average grant date fair value of nonvested awards at beginning of period	CAD 42.14
Weighted-average grant date fair value of units granted in period	53.19
Weighted-average grant date fair value of units vested in period	50.87
Weighted-average grant date fair value of units forfeited in period	37.62
Weighted-average grant date fair value of nonvested awards at end of period	CAD 44.18	CAD 42.14
Additional disclosures relating to stock-based compensation awards
Grant date fair value	CAD 23
Valuation method used in calculating the fair value of PSUs	Lattice-based valuation model
Unrecognized compensation cost	CAD 26
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition	1 year 7 months
Equity Settled Awards [Member] \| PSU-TSR [Member]
Roll forward of outstanding PSUs
Units outstanding at beginning of period	0.3
Units granted	0.1
Units settled	0.0
Units forfeited	0.0
Units outstanding at end of period	0.4	0.3
Weighted-average grant date fair value at beginning of period	CAD 103.93
Weighted-average grant date fair value of units granted in period	103.37
Weighted-average grant date fair value of units forfeited in period	97.06
Weighted-average grant date fair value at end of period	CAD 104.32	CAD 103.93
Roll forward of nonvested PSUs
Units nonvested at beginning of period	0.3
Units granted in period	0.1
Units vested during the year	(0.1)
Units forfeited in period	0.0
Units nonvested at end of period	0.3	0.3
Weighted-average grant date fair value of nonvested awards at beginning of period	CAD 103.93
Weighted-average grant date fair value of units granted in period	103.37
Weighted-average grant date fair value of units vested in period	114.86
Weighted-average grant date fair value of units forfeited in period	97.06
Weighted-average grant date fair value of nonvested awards at end of period	CAD 99.34	CAD 103.93
Additional disclosures relating to stock-based compensation awards
Grant date fair value	CAD 15
Valuation method used in calculating the fair value of PSUs	Monte Carlo simulation model
Unrecognized compensation cost	CAD 9
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition	1 year 7 months
Cash Settled Awards [Member] \| PSU-ROIC [Member]
Roll forward of outstanding PSUs
Units outstanding at beginning of period	0.4
Units granted	0.0
Units settled	(0.4)
Units forfeited	0.0
Units outstanding at end of period	0.0	0.4
Roll forward of nonvested PSUs
Units nonvested at beginning of period	0.0
Units granted in period	0.0
Units vested during the year	0.0
Units forfeited in period	0.0
Units nonvested at end of period	0.0	0.0
Additional disclosures relating to stock-based compensation awards
Share-based payment liability	CAD 0	CAD 45

The total grant date fair value of equity settled awards determined by using valuation models.

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The weighted average fair value at grant date for outstanding equity-based awards other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted-average grant date fair value of units settled during the period.

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Unrecognized cost of unvested share-based awards, other than options, awarded to employees as compensation.

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

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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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The weighted average fair value at grant date for nonvested equity-based awards issued 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).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

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

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For each plan, identification of the award pricing model or other valuation method used in calculating the weighted average fair values disclosed. The model is also used to calculate the compensation expense that is shown within the balance sheet, income statement, and cash flow. Examples of valuation techniques are lattice models (binomial model), closed-form models (Black-Scholes-Merton formula), and a Monte Carlo simulation technique. Fair value is the amount at which an asset or liability could be bought or incurred or sold or settled in a current transaction between willing parties, that is, other than in a forced or liquidation sale. May include disclosures about the assumptions underlying application of the method selected.

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Number of shares under non-option equity instrument agreements that were cancelled as a result of occurrence of a terminating event.

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Number of equity instruments other than options outstanding, including both vested and non-vested instruments.

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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)(1)(i)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(ii)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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