Income Taxes - Narrative (Details) - CAD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Unrecognized tax benefits from capital losses	$ 0	$ 0
Effect of tax rate decrease	(21)	(541)	$ 0
Income tax operating losses carried forward	8
Minimum tax credits carried forward	0
Investment tax credits carried forward	0
Net amount of accrued interest and penalties	0	1	1
Total amount of accrued interest and penalties associated with unrecognized tax benefit	$ 11	$ 11	$ 10

Amount before allocation of valuation allowances of deferred tax asset attributable to deductible capital loss carryforwards.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible alternative minimum tax credit carryforwards.

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The sum of the amounts of estimated penalties and interest recognized in the period arising from income tax examinations.

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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 operating loss carryforward, before tax effects, available to reduce future taxable income under enacted tax laws.

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Amount accrued for interest on an underpayment of income taxes and penalties related to a tax position claimed or expected to be claimed in the tax return.

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