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Amount of contractual obligation due after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

No definition available.

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Period Type:	instant

Amount of contractual obligation due in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

No definition available.

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Data Type:	xbrli:monetaryItemType
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Period Type:	instant

Amount of contractual obligation due in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

No definition available.

Name:	us-gaap_ContractualObligationDueInFourthYear
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Data Type:	xbrli:monetaryItemType
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Amount of contractual obligation due in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

No definition available.

Name:	us-gaap_ContractualObligationDueInNextTwelveMonths
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Amount of contractual obligation due in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

No definition available.

Name:	us-gaap_ContractualObligationDueInSecondYear
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Period Type:	instant

Amount of contractual obligation due in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

No definition available.

Name:	us-gaap_ContractualObligationDueInThirdYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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Period Type:	instant

Period covered by the long-term purchase commitment, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

Name:	us-gaap_LongtermPurchaseCommitmentPeriod
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Data Type:	xbrli:durationItemType
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Period Type:	duration

Reflects the estimated amount of loss from the specified contingency as of the balance sheet date.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118942415&loc=d3e14435-108349

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
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-URI http://asc.fasb.org/extlink&oid=118942415&loc=d3e14557-108349

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
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-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=118943654&loc=d3e12021-110248

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

If estimable, describe the recovery that could be expected to result from a right to proceed against a third party such as an insurer or co-defendant.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=118943654&loc=d3e12053-110248

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

The difference between the maximum amount of estimated loss and the amount recorded as of the balance sheet date.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 05
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=99385795&loc=d3e12631-108344

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Data Type:	xbrli:monetaryItemType
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Period Type:	instant

- References

No definition available.

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

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.

No definition available.

Name:	us-gaap_RecordedUnconditionalPurchaseObligationLineItems
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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Name:	srt_ConsolidatedEntitiesAxis=srt_SubsidiariesMember
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Data Type:	na
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Name:	srt_ConsolidatedEntitiesAxis=ema_TampaElectricDivisionMember
Namespace Prefix:
Data Type:	na
Balance Type:
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Name:	srt_ConsolidatedEntitiesAxis=ema_NovaScotiaPowerIncMember
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Data Type:	na
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Name:	srt_CounterpartyNameAxis=ema_NspMaritimeLinkIncMember
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Data Type:	na
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Name:	us-gaap_PublicUtilityAxis=ema_MaritimeLinkProjectMember
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Data Type:	na
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Name:	us-gaap_RecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis=us-gaap_ElectricityPurchasedMember
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Data Type:	na
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Name:	srt_ConsolidatedEntitiesAxis=ema_NewEnglandGasGeneratingFacilitiesMember
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Data Type:	na
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Name:	us-gaap_RecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis=us-gaap_OilAndGasServiceMember
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Data Type:	na
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Name:	us-gaap_RecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis=us-gaap_ConstructionContractsMember
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Data Type:	na
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Name:	us-gaap_PublicUtilityAxis=us-gaap_IndependentTransmissionAndDistributionSystemMember
Namespace Prefix:
Data Type:	na
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Name:	us-gaap_RecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis=ema_FuelAndGasSupplyMember
Namespace Prefix:
Data Type:	na
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Name:	us-gaap_RecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis=us-gaap_ServiceAgreementsMember
Namespace Prefix:
Data Type:	na
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Name:	us-gaap_RecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis=us-gaap_EquityMethodInvestmentsMember
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Data Type:	na
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Name:	us-gaap_RecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis=us-gaap_LeaseAgreementsMember
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Data Type:	na
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