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Income Taxes (Reasonable Probablity of Tax Benefit) (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued [Abstract]
Accrued interest	$ 1	$ 1
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Temporary Differences/Potential change	822	$ 667
Settlement with Taxing Authority [Member] | 2006 through 2010 years [Member] | Nova Scotia Power Inc. [Member] | Canada Revenue Agency [Member]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Net amount in dispute	62
Prepaid amount in dispute	$ 23

X

- Definition

Prepaid portion of estimated amount of loss resulting from an adverse tax position.

+ References

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Estimated amount of loss resulting from an adverse tax position.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (d)(3)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32718-109319

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- Definition

The amount of the unrecognized tax benefit of a position taken for which it is reasonably possible that the total amount thereof will significantly increase or decrease within twelve months of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32718-109319

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- Definition

Amount of interest expense accrued for an underpayment of income taxes.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)
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