Stock-based Compensation (VIDP) (Details) - CAD ($) $ / shares in Units, shares in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Weighted-average grant date fair value
Closing stock price (in dollars per share)	$ 101.11
Remittance of stock-based award participants' withholding tax obligation	$ 51,000,000	$ 57,000,000	$ 44,000,000
Voluntary Incentive Deferral Plan (VIDP) [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 (VIDP) [Member] \| Equity Settled Awards [Member] \| DSU [Member]
Roll forward of outstanding DSUs
Units outstanding at beginning of period (in shares)	1.1
Granted (in shares)	0.1
Settled (in shares)	(0.4)
Units outstanding at end of period (in shares)	0.8	1.1
Weighted-average grant date fair value
Weighted-average grant date fair value at beginning of period (in dollars per share)	$ 77.81
Weighted-average grant date fair value of units granted in period (in dollars per share)	95.64
Weighted-average grant date fair value of units settled in period (in dollars per share)	78.84
Weighted-average grant date fair value at end of period (in dollars per share)	$ 79.23	$ 77.81
Grant date fair value	$ 4,000,000
Aggregate intrinsic value of share units outstanding	78,000,000
Stock-based compensation liability outstanding	$ 19,000,000	$ 30,000,000
Fair value of awards vested during the period			0
Closing stock price (in dollars per share)	$ 101.11
Units settled (in shares)	0.2
Remittance of stock-based award participants' withholding tax obligation	$ 17,000,000
Voluntary Incentive Deferral Plan (VIDP) [Member] \| Cash Settled Awards [Member] \| DSU [Member]
Roll forward of outstanding DSUs
Units outstanding at beginning of period (in shares)	0.2
Granted (in shares)	0.0
Settled (in shares)	0.0
Units outstanding at end of period (in shares)	0.2	0.2
Weighted-average grant date fair value
Fair value of awards vested during the period	$ 0	$ 0	$ 0

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

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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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The number of units settled during the period.

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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.

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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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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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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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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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