Income Taxes - Narrative (Details) - CAD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Effect of change in enacted tax rate			$ (88)	$ (21)
Effective tax rate	25.43%	24.23%	21.06%	23.95%
Foreign exchange loss (gain) on debt and lease liabilities	$ 25	$ (38)	$ (57)	$ 55
Effective tax rate, excluding discrete items	25.11%	24.75%	25.50%	24.75%

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.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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