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Derivatives (Concentration Risk) (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Concentration Risk [Line Items]
Concentration Risk, Percentage	100.00%	100.00%
Concentration Risk	$ 1,394	$ 1,290
Accounts Receivable [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	81.00%	78.00%
Concentration Risk	$ 1,141	$ 1,014
Accounts Receivable [Member] | A- Rating [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	4.00%	4.00%
Concentration Risk	$ 55	$ 52
Accounts Receivable [Member] | BBB- to BBB+ Rating [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	4.00%	5.00%
Concentration Risk	$ 61	$ 60
Accounts Receivable [Member] | Not Rated [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	7.00%	4.00%
Concentration Risk	$ 96	$ 57
Accounts Receivable, Residential [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	23.00%	24.00%
Concentration Risk	$ 326	$ 315
Accounts Receivable, Commercial [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	11.00%	13.00%
Concentration Risk	$ 161	$ 170
Accounts Receivable, Industrial [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	3.00%	3.00%
Concentration Risk	$ 46	$ 38
Accounts Receivable, Other [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	7.00%	5.00%
Concentration Risk	$ 96	$ 69
Regulated utilities [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	44.00%	45.00%
Concentration Risk	$ 629	$ 592
Trading group [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	15.00%	13.00%
Concentration Risk	$ 212	$ 169
Other Accounts Receivable [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	22.00%	20.00%
Concentration Risk	$ 300	$ 253
Derivative [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	19.00%	22.00%
Concentration Risk	$ 253	$ 276
Derivative [Member] | A- Rating [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	15.00%	20.00%
Concentration Risk	$ 207	$ 252
Derivative [Member] | BBB- to BBB+ Rating [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	1.00%	0.00%
Concentration Risk	$ 10	$ 1
Derivative [Member] | Not Rated [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	3.00%	2.00%
Concentration Risk	$ 36	$ 23

X

- Definition

Maximum amount of loss due to credit risk that, based on the gross fair value of the financial instrument, the entity would incur if parties to the financial instruments that make up the concentration failed completely to perform according to the terms of the contracts and the collateral or other security, if any, for the amount due proved to be of no value to the entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13537-108611

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6351-108592

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13531-108611

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6404-108592

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
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-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13537-108611

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