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Investments Subject to Significant Influence and Equity Income (Equity Method) (Details) - CAD shares in Millions, CAD in Millions | 3 Months Ended | 6 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Dec. 31, 2016 | Dec. 31, 2015 | Jun. 30, 2016 | Dec. 31, 2016 | Dec. 31, 2015 | May 24, 2016
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Equity Method Investments | CAD 947 | CAD 1,145 |  | CAD 947 | CAD 1,145 |
Income (Loss) from Equity Method Investments |  |  |  | CAD 100 | 108 |
Equity Method Investment, Summarized Financial Information [Abstract] |  |  |  |  |  |
Indirect Ownership Percentage By Parent | 100.00% |  |  | 100.00% |  |
Equity Method Investments | CAD 947 | 1,145 |  | CAD 947 | 1,145 |
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity | 14 |  |  | 14 |  |
Other Liabilities, Noncurrent | 467 | 299 |  | 467 | 299 |
Labrador-Island Link Limited Partnership [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Equity Method Investments | CAD 400 | 208 |  | 400 | 208 |
Income (Loss) from Equity Method Investments |  |  |  | CAD 24 | 9 |
Equity Method Investment, Ownership Percentage | 62.70% |  |  | 62.70% |  |
Equity Method Investment, Summarized Financial Information [Abstract] |  |  |  |  |  |
Indirect Ownership Percentage By Parent | 24.90% |  |  | 24.90% |  |
Equity Method Investments | CAD 400 | 208 |  | CAD 400 | 208 |
NSP Maritime Link Inc. [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Equity Method Investments | CAD 315 | 188 |  | 315 | 188 |
Income (Loss) from Equity Method Investments |  |  |  | CAD 21 | 15 |
Equity Method Investment, Ownership Percentage | 100.00% |  |  | 100.00% |  |
Equity Method Investment, Summarized Financial Information [Abstract] |  |  |  |  |  |
Equity Method Investments | CAD 315 | 188 |  | CAD 315 | 188 |
Maritimes and Northeast Pipline [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Equity Method Investments | CAD 175 | 189 |  | 175 | 189 |
Income (Loss) from Equity Method Investments |  |  |  | CAD 23 | 23 |
Equity Method Investment, Ownership Percentage | 12.90% |  |  | 12.90% |  |
Equity Method Investment, Summarized Financial Information [Abstract] |  |  |  |  |  |
Equity Method Investments | CAD 175 | 189 |  | CAD 175 | 189 |
Saint Lucia Electricity Services Limited [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Equity Method Investments | CAD 39 | 39 |  | 39 | 39 |
Income (Loss) from Equity Method Investments |  |  |  | CAD 3 | 3 |
Equity Method Investment, Ownership Percentage | 19.10% |  |  | 19.10% |  |
Equity Method Investment, Summarized Financial Information [Abstract] |  |  |  |  |  |
Equity Method Investments | CAD 39 | 39 |  | CAD 39 | 39 |
Algonquin Power and Utilities Corporation [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Equity Method Investments | CAD 0 | 504 |  | 0 | 504 |
Income (Loss) from Equity Method Investments |  |  |  | CAD 18 | 37 |
Equity Method Investment, Ownership Percentage | 0.00% |  |  | 0.00% |  |
Equity Method Investment, Summarized Financial Information [Abstract] |  |  |  |  |  |
Indirect Ownership Percentage By Parent |  |  |  |  |  | 19.30%
Common Stock Sold Equity Method Investments |  |  |  |  |  | 50.1
Gain (Loss) on Sale of Equity Investments | CAD (12) |  | CAD 172 |  |  |
After Tax Gain Loss On Sale Of Equity Investments | (10) |  | CAD 146 |  |  |
Subscription Receipts And Dividend Equivalents Exchanged For Common Shares |  |  | 12.9 |  |  |
Equity Method Investments | 0 | 504 |  | CAD 0 | 504 |
Bear Swamp Power Company Limited Liability Company [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Equity Method Investments | CAD 0 | 0 |  | 0 | 0 |
Income (Loss) from Equity Method Investments |  |  |  | CAD 11 | 17 |
Equity Method Investment, Ownership Percentage | 50.00% |  |  | 50.00% |  |
Equity Method Investment, Summarized Financial Information [Abstract] |  |  |  |  |  |
Proceeds from Equity Method Investment, Dividends or Distributions, Return of Capital |  | 179 |  |  |  |
Equity Method Investments | CAD 0 | 0 |  | CAD 0 | 0 |
Other Liabilities, Noncurrent | 217 | 225 |  | 217 | 225 |
APUC Subscription Receipts Dividend Equivalents Common Shares [Member] |  |  |  |  |  |
Equity Method Investment, Summarized Financial Information [Abstract] |  |  |  |  |  |
Gain (Loss) on Sale of Equity Investments |  |  | CAD 63 |  |  |
After Tax Gain Loss On Sale Of Equity Investments |  |  | CAD 53 |  |  |
Other Investments |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Equity Method Investments | 18 | 17 |  | 18 | 17 |
Income (Loss) from Equity Method Investments |  |  |  | 0 | 4 |
Equity Method Investment, Summarized Financial Information [Abstract] |  |  |  |  |  |
Equity Method Investments | CAD 18 | CAD 17 |  | CAD 18 | CAD 17 |

X
- Definition The after tax difference between the carrying value and the sale price of equity securities, not separately or otherwise categorized as trading or available-for-sale. This element includes investments in which the entity holds a small ownership stake (generally, less than 20% of the shares outstanding) and cannot exert significant influence. + References No definition available. + Details
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- Definition Difference between amount at which an investment accounted for under the equity method of accounting is carried (reported) on the balance sheet and amount of underlying equity in net assets the reporting Entity has in the investee. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=66011266&loc=d3e33918-111571 + Details
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- Definition This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=66022249&loc=d3e33749-111570 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.12) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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