Income Taxes - Narrative (Details) - CAD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Tax Disclosure [Abstract]
Tax recovery due to changes in income tax rates	$ (21)	$ (17)	$ (21)	$ (17)
Effective tax rate	21.88%	22.31%	23.73%	23.90%
Foreign exchange loss (gain) on long-term debt	$ 44	$ (67)	$ 93	$ (95)
Effective tax rate, excluding discrete items	24.75%	26.50%	24.75%	26.50%
Insurance recovery of legal settlement	$ 0	$ (10)	$ 0	$ (10)
Charge on hedge roll and de-designation	$ 0	$ 13	$ 0	13
Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation and benefits				$ (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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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 the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations, attributable to increase (decrease) in the income tax rates.

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

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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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