Stock-based Compensation (VIDP) (Details) - CAD CAD / shares in Units, shares in Millions, CAD in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Weighted-average grant date fair value
Closing stock price	CAD 103.65
Remittance of stock-based award participants' withholding tax obligation	CAD 57	CAD 44	CAD 2
Voluntary Incentive Deferral Plan [Member] \| DSU [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Company's additional grant percentage of elected amount	25.00%
The number of years in the plan period over which the additional units will vest	4 years
Voluntary Incentive Deferral Plan [Member] \| Equity Settled Awards [Member] \| DSU [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of awards vested during the period	CAD 1	CAD 1	1
Roll forward of outstanding DSUs
Units outstanding at beginning of period	1.5
Granted	0.1
Vested	0.0
Settled	(0.5)
Units outstanding at end of period	1.1	1.5
Weighted-average grant date fair value
Weighted-average grant date fair value at beginning of period	CAD 76.54
Weighted-average grant date fair value of units granted in period	93.85
Weighted-average grant date fair value of units vested in period	97.23
Weighted-average grant date fair value of units settled in period	76.54
Weighted-average grant date fair value at end of period	CAD 77.81	CAD 76.54
Grant date fair value	CAD 4
Aggregate intrinsic value of share units outstanding	CAD 105
Units settled	0.3
Remittance of stock-based award participants' withholding tax obligation	CAD 27
Voluntary Incentive Deferral Plan [Member] \| Cash Settled Awards [Member] \| DSU [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Description of the method used for calculating number of awards under the plan	For cash settled DSUs, the number of DSUs received by each participant is calculated using the Company’s average closing share price for the 20 trading days prior to and including the date of the incentive payment.
Fair value of awards vested during the period	CAD 0	CAD 0	CAD 0
Roll forward of outstanding DSUs
Units outstanding at beginning of period	0.3
Granted	0.0
Vested	0.0
Settled	(0.1)
Units outstanding at end of period	0.2	0.3
Weighted-average grant date fair value
Stock-based compensation liability outstanding	CAD 30	CAD 35
Closing stock price	CAD 103.65

Percentage of additional units granted by Company based on the amount elected by participant.

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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 stock-based compensation liability outstanding related to deferred share units.

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Description of the method used for calculating number of awards under the plan.

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The number of units settled during the period.

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- Definition

Amount of cash outflow to satisfy an employee's income tax withholding obligation as part of a net-share settlement of a share-based award.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (a)
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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.

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

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Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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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.

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

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Intrinsic value of equity-based compensation awards outstanding. Excludes stock and unit options.

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