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Derivatives (Concentration Risk) (Details) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Concentration Risk [Line Items]
Concentration Risk, Percentage	100.00%	100.00%
Concentration Risk	CAD 1,290	CAD 996
Accounts Receivable [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	78.00%	58.00%
Concentration Risk	CAD 592	CAD 374
Accounts Receivable [Member] | A- Rating [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	4.00%	3.00%
Concentration Risk	CAD 52	CAD 31
Accounts Receivable [Member] | BBB- to BBB+ Rating [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	5.00%	2.00%
Concentration Risk	CAD 60	CAD 22
Accounts Receivable [Member] | Not Rated [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	4.00%	3.00%
Concentration Risk	CAD 57	CAD 31
Accounts Receivable, Residential [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	24.00%	20.00%
Concentration Risk	CAD 315	CAD 189
Accounts Receivable, Commercial [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	13.00%	10.00%
Concentration Risk	CAD 170	CAD 103
Accounts Receivable, Industrial [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	3.00%	3.00%
Concentration Risk	CAD 38	CAD 29
Accounts Receivable, Other [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	5.00%	5.00%
Concentration Risk	CAD 69	CAD 53
Regulated utilities [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	45.00%	38.00%
Concentration Risk	CAD 592	CAD 374
Trading group [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	13.00%	8.00%
Concentration Risk	CAD 169	CAD 84
Other Accounts Receivable [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	20.00%	12.00%
Concentration Risk	CAD 253	CAD 120
Derivative [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	22.00%	42.00%
Concentration Risk	CAD 276	CAD 418
Derivative [Member] | A- Rating [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	20.00%	34.00%
Concentration Risk	CAD 252	CAD 340
Derivative [Member] | BBB- to BBB+ Rating [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	0.00%	7.00%
Concentration Risk	CAD 1	CAD 70
Derivative [Member] | Not Rated [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	2.00%	1.00%
Concentration Risk	CAD 23	CAD 8

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=73719730&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=64851502&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=73719730&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=64851502&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
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13537-108611

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