Stock-based compensation (PSU Table) (Details 3) - Share Units Plan [Member] - CAD CAD / shares in Units, shares in Millions, CAD in Millions	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015
Equity settled awards \| PSUs-ROIC
Roll forward of the activity of performance share units outstanding
Outstanding units at beginning of period	1.3
Units granted during the period (in shares)	0.5
Units settled during the period (in shares)	(0.5)
Outstanding units at end of period	1.3	1.3
Weighted-average grant date fair value outstanding units beginning of period	CAD 64.36
Weighted-average grant date fair value of units granted in period	35.11
Weighted-average grant date fair value of units settled during the period	75.15
Weighted-average grant date fair value outstanding units end of period	CAD 49.82	CAD 64.36
Roll forward of the activity of performance share units nonvested
Nonvested at beginning of the period (in shares)	0.8
Share-basedCompensation Arrangement by Share-based Payment Award, Equity Instruments Other Than Options Grants in Period	0.5
Vested during the year (in shares)	(0.4)
Nonvested at end of the period (in shares)	0.9	0.8
Weighted-average grant date fair value of nonvested awards beginning of period	CAD 58.83
Weighted-average grant date fair value of units granted in period	35.11
Weighted-average grant date fair value of units vested during the period	66.84
Weighted-average grant date fair value of nonvested awards end of period	CAD 42.14	CAD 58.83
Additional disclosures relating to stock-based compensation awards
The grant date fair value of equity settled awards	CAD 19
Valuation method used in calculating the fair value of performance share units	Lattice-based valuation model
Unrecognized compensation cost	CAD 15
Weighted-average period of recognition for unrecognized compensation costs	1 year 6 months
Equity settled awards \| PSUs-TSR
Roll forward of the activity of performance share units outstanding
Outstanding units at beginning of period	0.1
Units granted during the period (in shares)	0.2
Units settled during the period (in shares)	0.0
Outstanding units at end of period	0.3	0.1
Weighted-average grant date fair value outstanding units beginning of period	CAD 114.86
Weighted-average grant date fair value of units granted in period	95.31
Weighted-average grant date fair value outstanding units end of period	CAD 103.93	CAD 114.86
Roll forward of the activity of performance share units nonvested
Nonvested at beginning of the period (in shares)	0.1
Share-basedCompensation Arrangement by Share-based Payment Award, Equity Instruments Other Than Options Grants in Period	0.2
Vested during the year (in shares)	0.0
Nonvested at end of the period (in shares)	0.3	0.1
Weighted-average grant date fair value of nonvested awards beginning of period	CAD 114.86
Weighted-average grant date fair value of units granted in period	95.31
Weighted-average grant date fair value of nonvested awards end of period	CAD 103.93	CAD 114.86
Additional disclosures relating to stock-based compensation awards
The grant date fair value of equity settled awards	CAD 17
Valuation method used in calculating the fair value of performance share units	Monte Carlo simulation model
Unrecognized compensation cost	CAD 10
Weighted-average period of recognition for unrecognized compensation costs	1 year 6 months
Cash settled awards \| PSUs-ROIC
Roll forward of the activity of performance share units outstanding
Outstanding units at beginning of period	0.7
Units granted during the period (in shares)	0.0
Units settled during the period (in shares)	(0.3)
Outstanding units at end of period	0.4	0.7
Roll forward of the activity of performance share units nonvested
Nonvested at beginning of the period (in shares)	0.4
Share-basedCompensation Arrangement by Share-based Payment Award, Equity Instruments Other Than Options Grants in Period	0.0
Vested during the year (in shares)	(0.4)
Nonvested at end of the period (in shares)	0.0	0.4
Additional disclosures relating to stock-based compensation awards
Valuation method used in calculating the fair value of performance share units	Lattice-based valuation model
Share-based payment liability at period end	CAD 45	CAD 66

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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Period end liability associated with share-based compensation (cash-settled awards)

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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.

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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 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).

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- Definition

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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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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Net number of non-option equity instruments granted to participants.

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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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