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Investments Subject to Significant Influence and Equity Income (Equity Method) (Details) - CAD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2015 | Dec. 31, 2017 | Dec. 31, 2016
Schedule of Equity Method Investments [Line Items] |  |  |
Equity Method Investments |  | $ 1,215 | $ 947
Income (Loss) from Equity Method Investments |  | 124 | 100
Equity Method Investment, Summarized Financial Information [Abstract] |  |  |
Equity Method Investments |  | 1,215 | 947
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity |  | 13 |
Other Liabilities, Noncurrent |  | 609 | 645
Labrador-Island Link Limited Partnership [Member] |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Equity Method Investments |  | 492 | 400
Income (Loss) from Equity Method Investments |  | $ 37 | 24
Equity Method Investment, Ownership Percentage |  | 49.50% |
Equity Method Investment, Summarized Financial Information [Abstract] |  |  |
Indirect Ownership Percentage By Parent |  | 24.90% |
Equity Method Investments |  | $ 492 | 400
NSP Maritime Link Inc. [Member] |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Equity Method Investments |  | 510 | 315
Income (Loss) from Equity Method Investments |  | $ 36 | 21
Equity Method Investment, Ownership Percentage |  | 100.00% |
Equity Method Investment, Summarized Financial Information [Abstract] |  |  |
Equity Method Investments |  | $ 510 | 315
Maritimes and Northeast Pipline [Member] |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Equity Method Investments |  | 156 | 175
Income (Loss) from Equity Method Investments |  | $ 23 | 23
Equity Method Investment, Ownership Percentage |  | 12.90% |
Equity Method Investment, Summarized Financial Information [Abstract] |  |  |
Equity Method Investments |  | $ 156 | 175
Saint Lucia Electricity Services Limited [Member] |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Equity Method Investments |  | 39 | 39
Income (Loss) from Equity Method Investments |  | $ 3 | 3
Equity Method Investment, Ownership Percentage |  | 19.10% |
Equity Method Investment, Summarized Financial Information [Abstract] |  |  |
Equity Method Investments |  | $ 39 | 39
Algonquin Power and Utilities Corporation [Member] |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Income (Loss) from Equity Method Investments |  | 0 | 18
Bear Swamp Power Company Limited Liability Company [Member] |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Equity Method Investments |  | 0 | 0
Income (Loss) from Equity Method Investments |  | $ 23 | 11
Equity Method Investment, Ownership Percentage |  | 50.00% |
Equity Method Investment, Summarized Financial Information [Abstract] |  |  |
Proceeds from Equity Method Investment, Dividends or Distributions, Return of Capital | $ 179 |  |
Equity Method Investments |  | $ 0 | 0
Other Liabilities, Noncurrent |  | 188 | 217
Other Investments |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Equity Method Investments |  | 18 | 18
Income (Loss) from Equity Method Investments |  | 2 | 0
Equity Method Investment, Summarized Financial Information [Abstract] |  |  |
Equity Method Investments |  | $ 18 | $ 18

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