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Regulatory Assets and Liabilities (ROE of New Mexico Gas Company) (Details) CAD in Millions	12 Months Ended
	Dec. 31, 2016 CAD
New Mexico Gas Company [Member] | New Mexico Public Regulatory [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities Bill Reduction	CAD 4.0

X

- Definition

Amount of public utility's annual credit issued to customers while waiting for the effective base rate increase date.

+ References

No definition available.

+ Details

Name:	ema_PublicUtilitiesBillReduction
Namespace Prefix:	ema_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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

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

X

- Details

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

X

- Details

Name:	us-gaap_PublicUtilitiesRegulatoryProceedingAxis=ema_NewMexicoPublicRegulatoryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: