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Risk Management Activities - Non-Hedges (Details) - CAD CAD in Millions	12 Months Ended
	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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The gains (losses) resulting from change in the fair value of derivatives recognised in profit or loss. [Refer: Derivatives [member]]

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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]]

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