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Derivatives (Master Netting Agreements) (Details) - CAD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Derivative Asset, Collateral, Obligation to Return Cash, Offset	$ 181	$ 131
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract]
Derivative Liability, Collateral, Right to Reclaim Cash, Offset	181	131
Designated as Hedging Instrument [Member] | Regulatory Deferral Hedge [Member]
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Derivative Asset, Collateral, Obligation to Return Cash, Offset	14	10
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract]
Derivative Liability, Collateral, Right to Reclaim Cash, Offset	14	10
Designated as Hedging Instrument [Member] | Held for Trading Hedge [Member]
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Derivative Asset, Collateral, Obligation to Return Cash, Offset	167	121
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract]
Derivative Liability, Collateral, Right to Reclaim Cash, Offset	$ 167	$ 121

X

- Definition

Amount of obligation to return cash collateral under master netting arrangements offset against derivative assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Amount of right to receive cash collateral under master netting arrangements offset against derivative liabilities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

+ Details

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Period Type:	instant

X

- References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
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- Details

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- Details

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- Details

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