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Income Taxes (Narrative) (Details) - CAD ($) |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
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 |

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