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Regulatory Assets and Liabilities (ROE of New Mexico Gas Company) (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2017 CAD ($)
New Mexico Gas Company [Member] | New Mexico Public Regulatory [Member] |
Public Utilities, General Disclosures [Line Items] |
Public Utilities Bill Reduction | $ 4.0

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- 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
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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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. + Details
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Namespace Prefix: | us-gaap_
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Period Type: | duration

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- Details
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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: |