Income Taxes - Narrative (Details) - CAD CAD in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2017	Jun. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Tax Disclosure [Abstract]
Tax Adjustments, Settlements, and Unusual Provisions	CAD 3	CAD (17)		CAD (14)
Foreign exchange (gain) loss on long-term debt	(105)		CAD 46	(200)	CAD (153)
Legal settlement	0		25	0	25
Insurance recovery of legal settlement	0		0	(10)	0
Charge on hedge roll and de-designation	CAD 0		CAD 0	CAD 13	CAD 0
Effective tax rate	24.95%		26.23%	24.28%	25.26%
Estimated annual effective tax rate	26.50%		25.17%	26.50%	26.50%
Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation and benefits				CAD (51)

Effective Income Tax Rate Reconciliation, Estimated Rate, Excluding Discrete Items, Percent

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The aggregate foreign currency transaction gain or loss on long-term debt (both realized and unrealized) included in determining net income for the reporting period net of hedging activity.

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Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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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.

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- Definition

The difference between the book value and the sale price of options, swaps, futures, forward contracts, and other derivative instruments. This element refers to the gain (loss) included in earnings.

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Amount of excess (deficiency) of insurance recovery over the loss incurred from an insured event.

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The amount of expense provided in the period for legal costs incurred on or before the balance sheet date pertaining to resolved, pending or threatened litigation, including arbitration and mediation proceedings.

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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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Amount of increase (decrease) to previously recorded tax expense. Includes, but is not limited to, significant settlements of income tax disputes, and unusual tax positions or infrequent actions taken by the entity, including tax assessment reversal, and IRS tax settlement.

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