Income Taxes - Narrative (Details) - CAD	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Unrecognized tax benefits from capital losses	CAD 0	CAD 0
Effect of tax rate (decrease) increase	(541,000,000)	0	CAD 23,000,000
Income tax operating losses carried forward	11,000,000
Minimum tax credits	0
Investment tax credits	0
Total amount of accrued interest and penalties	1,000,000	1,000,000	4,000,000
The total amount of accrued interest and penalties associated with unrecognized tax benefit	CAD 11,000,000	CAD 10,000,000	CAD 9,000,000

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32621-109319

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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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