Prepaid portion of estimated amount of loss resulting from an adverse tax position.

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Income Tax Expense, Revaluation Of Certain Net Deferred Income Tax Assets

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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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Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

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Percentage 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 applicable to state and local income tax expense (benefit), net of federal tax expense (benefit).

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Estimated amount of loss resulting from an adverse tax position.

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The amount of estimated interest recognized in the period arising from income tax examinations.

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The increase (decrease) during the reporting period in the account that represents the temporary difference that results from Income or Loss that is recognized for accounting purposes but not for tax purposes and vice versa.

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Tax year that remains open to examination under enacted tax laws, in CCYY format.

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The amount of the unrecognized tax benefit of a position taken for which it is reasonably possible that the total amount thereof will significantly increase or decrease within twelve months of the balance sheet date.

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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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Amount of statutory penalties accrued for a tax position claimed or expected to be claimed by the entity, in its tax return.

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Amount of interest expense accrued for an underpayment of income taxes.

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The total amount of unrecognized tax benefits that, if recognized, would affect the effective tax rate.

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Income Taxes (Narrative) (Details) - CAD ($)		3 Months Ended	12 Months Ended
Income Taxes (Narrative) (Details) - CAD ($)	Mar. 09, 2020	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Entity [Domain]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Temporary Differences/Potential change			$ 2,700,000,000	$ 1,900,000,000
Decrease in deferred income tax liablities		$ 52,000,000
Income tax expense, revaluation of certain net deferred income tax assets		12,000,000
Deferred Tax Assets, Valuation Allowance			202,000,000	193,000,000
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued [Abstract]
Amount that could affect effective tax rate			30,000,000	29,000,000
Accrued interest			6,000,000	5,000,000
Income Tax Examination, Interest Expense			1,000,000	$ 1,000,000
Accrued penalties			0
Entity [Domain] \| Deferred income tax assets \| Revision of Prior Period, Adjustment for Reclassification
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
AMT credit carryforward		(77,000,000)
Entity [Domain] \| Receivables and other current assets
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
AMT credit carryforward			$ 145,000,000
Entity [Domain] \| Receivables and other current assets \| Revision of Prior Period, Adjustment for Reclassification
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
AMT credit carryforward		$ 77,000,000
Entity [Domain] \| Canada Revenue Agency
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Local income tax rate	31.00%		29.50%
Entity [Domain] \| Canada Revenue Agency \| Nova Scotia
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Local income tax rate	16.00%		14.00%
2005 and subsequent years \| Entity [Domain]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Open Tax Year			2005
Subsequent years after 2020 \| Entity [Domain] \| Canada Revenue Agency
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Local income tax rate			29.00%
Settlement with Taxing Authority \| 2006 through 2010 years \| Nova Scotia Power Inc. \| Canada Revenue Agency
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Net amount in dispute			$ 62,000,000
Prepaid amount in dispute			$ 23,000,000