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Interest Expense, Net (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Interest on debt	$ 663	$ 443
Beneficial Conversion Feature, net of tax	0	(43)
Beneficial Conversion Feature, before tax	0	62
Allowance for Funds Used During Construction	(7)	(13)
Other	42	17
Interest Expense (Income), Net	698	585
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Interest on debt	0	11
Convertible Debt [Member]
Debt Instrument [Line Items]
Interest on debt	$ 0	$ 65

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- Definition

Debt Instrument, Convertible, Beneficial Conversion Feature before tax

+ References

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- Definition

Amount of a favorable spread to a debt holder between the amount of debt being converted and the value of the securities received upon conversion. This is an embedded conversion feature of convertible debt issued that is in-the-money at the commitment date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21538-112644

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Amount of the cost of borrowed funds accounted for as interest expense for debt.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.8)
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Amount of interest expense classified as other.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4D
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624177-113959

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-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph -4
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

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The net amount of operating interest income (expense).

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-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
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Amount capitalized of allowance for funds used during construction.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 835
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6501569&loc=d3e55921-110430

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