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Derivatives (Credit Risk Exposure) (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Credit Derivatives [Line Items]
Concentration Risk, Credit Risk, Financial Instrument, Maximum Exposure	$ 1,394	$ 1,290
Total cash deposits/collateral on hand	119	91
Accounts Receivable [Member]
Credit Derivatives [Line Items]
Concentration Risk, Credit Risk, Financial Instrument, Maximum Exposure	1,148	1,019
Total cash deposits/collateral on hand	247	271
Financial assets, considered to be past due	$ 90	104
Derivative, Credit Risk, Past Due Days Outstanding	69 days
Fair Value of assets	$ 78	$ 91

X

- Definition

The average days outstanding on financial assets related to derivatives expressed as PnYnMnDTnHnMnS.

+ References

No definition available.

+ Details

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X

- Definition

Amount of finacial assets that are past due related to Derivatives.

+ References

No definition available.

+ Details

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X

- Definition

Net amount after allowance for doubtful accounts of finacial assets that are past due related to Derivatives.

+ References

No definition available.

+ Details

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Namespace Prefix:	ema_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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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X

- 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.

+ Details

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X

- Definition

Amount of obligation to return cash collateral under master netting arrangements that have not been 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 50
-Paragraph 8
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=d3e41678-113959

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

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226016-175313

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