Income Taxes (Narrative) (Details) - CAD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Entity [Domain]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Temporary Differences/Potential change	$ 1,900	$ 1,400
2005 and subsequent years \| Entity [Domain]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Open Tax Year	2005
Settlement with Taxing Authority \| 2006 through 2010 years \| Nova Scotia Power Inc. \| Canada Revenue Agency
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Net amount in dispute	$ 62
Prepaid amount in dispute	$ 23

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

No definition available.

Name:	ema_IncomeTaxExaminationPrepaymentOfPossibleLoss
Namespace Prefix:	ema_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Estimated amount of loss resulting from an adverse tax position.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-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

+ Details

Name:	us-gaap_IncomeTaxExaminationEstimateOfPossibleLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Tax year that remains open to examination under enacted tax laws, in CCYY format.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32718-109319

+ Details

Name:	us-gaap_OpenTaxYear
Namespace Prefix:	us-gaap_
Data Type:	us-types:gYearListItemType
Balance Type:	na
Period Type:	duration

- 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://fasb.org/us-gaap/role/ref/legacyRef
-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

+ Details

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

- 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

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

- Details

Name:	dei_LegalEntityAxis=dei_EntityDomain
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TaxPeriodAxis=ema_TwoThousandFiveAndSubsequentYearsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ReasonablyPossibleSignificantChangeInUnrecognizedTaxBenefitsByItemAxis=us-gaap_SettlementWithTaxingAuthorityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TaxPeriodAxis=ema_TwoZeroZeroSixToTwoZeroOneZeroMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=ema_NovaScotiaPowerIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeTaxAuthorityNameAxis=us-gaap_CanadaRevenueAgencyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: