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Risk Management Activities - Non-Hedges (Details) - CAD CAD in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |
Foreign exchange gain (loss) | CAD (1) | CAD (5) | CAD 4
Gains (losses) on change in fair value of derivatives | 45 | 63 | 51
Gains on change in fair value of derivatives | 28 | 9 | (1)
Unrealised Loss On Change In Fair Value Of Derivatives | (2) | 4 | (11)
Unrealised Gains On Change In Fair Value Of Derivatives | 30 | 5 | CAD 10
Currency risk |  |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |
Foreign exchange gain (loss) | CAD (6) | CAD (5) |

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- Definition The gains (losses) resulting from change in the fair value of derivatives recognised in profit or loss. [Refer: Derivatives [member]] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 1 -IssueDate 2017-01-01 -Paragraph 85 -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_85&doctype=Standard -URIDate 2017-03-09 + Details
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- Definition The amount of exchange differences recognised in profit or loss that arise from foreign currency transactions, excluding those arising on financial instruments measured at fair value through profit or loss in accordance with IFRS 9. [Refer: At fair value [member]; Financial instruments, class [member]] + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Name IAS -Number 7 -IssueDate 2017-01-01 -Section A Statement of cash flows for an entity other than a financial institution -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=7&date=2017-03-01&anchor=sect_AStatementofcashflowsforanentityotherthanafinancialinstitution&doctype=Illustrative%20Examples -URIDate 2017-03-09 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 21 -IssueDate 2017-01-01 -Paragraph 52 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=21&date=2017-03-01&anchor=para_52_a&doctype=Standard -URIDate 2017-03-09 + Details
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