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Risk Management Activities - Foreign Currency Rate Risk Management (Details) - CAD CAD in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2017		Dec. 31, 2016	[1]	Dec. 31, 2015	[1]
Financial Instruments [Abstract]
Derivative hedge, notional amount		CAD 690
Net gains on derivatives designated as cash flow hedges, net of tax	CAD 3	CAD 214	[1]	CAD 179		CAD 375
[1] (7) Net of income tax recovery of 77 for the year ended Dec. 31, 2017 (2016 - 92 expense, 2015 - 138 expense).

X

- Definition

The gains (losses) recognised in other comprehensive income on cash flow hedges, net of tax. [Refer: Cash flow hedges [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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-Name IFRS
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X

- Definition

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

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- Definition

Financial Instruments [Abstract]

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No definition available.

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