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Financial Instruments - Summary of Earnings Impact of (Gains) Losses from Risk Management Positions (Detail) - CAD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Disclosure Of Financial Instruments [Abstract] |  |  |
Realized (Gain) Loss | $ 252 | $ 7 | $ 1,554
Unrealized (Gain) Loss | 56 | 149 | (1,249)
(Gain) Loss on Risk Management From Continuing Operations | $ 308 | $ 156 | $ 305

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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 2020-01-01 -Paragraph 85 -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2020-en-r&anchor=para_85&doctype=Standard -URIDate 2020-01-01 + Details
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