Income Taxes - Reconciliation of Income Taxes Calculated at Statutory Rate (Detail) - CAD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation Of Average Effective Tax Rate And Applicable Tax Rate [Abstract]
Earnings (Loss) From Continuing Operations Before Income Tax	$ (3,926)	$ 2,216	$ (802)
Canadian Statutory Rate	27.00%	27.00%	27.00%
Expected Income Tax Expense (Recovery) From Continuing Operations	$ (1,060)	$ 598	$ (217)
Effect of Taxes Resulting From:
Foreign Tax Rate Differential	(57)	(17)	(46)
Non-Taxable Capital (Gains) Losses	82	(129)	(26)
Non-Recognition of Capital (Gains) Losses	99	(99)	(26)
Adjustments Arising From Prior Year Tax Filings	3	(41)	(46)
Recognition of Previously Unrecognized Capital Losses		(68)
Recognition of U.S. Tax Basis	(78)
Change in Statutory Rate		(275)
Non-Deductible Expenses	2	(5)	5
Other	(1)	(16)	13
Tax Expense (Recovery) From Continuing Operations	$ (1,010)	$ (52)	$ (343)
Effective Tax Rate	25.70%	(2.30%)	42.80%

Adjustments of tax expense (income) recognized in the period for tax of prior periods.

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Deferred tax impact of adjustment to US tax basis.

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Source of origin of tax rate differential.

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Tax effect of non-recognition of capital (gains) losses.

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The amount of profit (loss) for a period before deducting tax expense. [Refer: Profit (loss)]

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The applicable income tax rate.

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The tax expense (income) divided by the accounting profit. [Refer: Accounting profit]

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The aggregate amount included in the determination of profit (loss) for the period in respect of current tax and deferred tax that relate to continuing operations. [Refer: Continuing operations [member]; Current tax expense (income); Deferred tax expense (income)]

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The amount that represents the difference between the tax expense (income) and the product of the accounting profit multiplied by the applicable tax rate(s) that the entity does not separately disclose in the same statement or note. [Refer: Accounting profit; Applicable tax rate]

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The amount that represents the difference between the tax expense (income) and the product of the accounting profit multiplied by the applicable tax rate(s) that relates to changes in the tax rate. [Refer: Accounting profit]

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The amount that represents the difference between the tax expense (income) and the product of the accounting profit multiplied by the applicable tax rate(s) that relates to expenses not deductible in determining taxable profit (tax loss). [Refer: Accounting profit]

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The amount that represents the difference between the tax expense (income) and the product of the accounting profit multiplied by the applicable tax rate(s) that relates to foreign tax rates. [Refer: Accounting profit]

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The amount that represents the difference between the tax expense (income) and the product of the accounting profit multiplied by the applicable tax rate(s) that relates to revenues that are exempt from taxation. [Refer: Accounting profit]

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The amount that represents the difference between the tax expense (income) and the product of the accounting profit multiplied by the applicable tax rate(s) that relates to tax losses. [Refer: Accounting profit]

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The product of the accounting profit multiplied by the applicable tax rate(s). [Refer: Accounting profit; Applicable tax rate]

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