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Long Term Debt (Debt Covenants) (Details) - Syndicated Credit Facilities [Member] | Dec. 31, 2016
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Debt Instrument [Line Items] |
Debt Instrument Covenant Requirement | 0.7
Ratio of Indebtedness to Net Capital | 0.62

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- Definition Coveneant requirement for debt instrument. + References No definition available. + Details
Name: | ema_DebtInstrumentCovenantRequirement
Namespace Prefix: | ema_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | instant

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

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- Definition Indebtedness divided by net capital. + References No definition available. + Details
Name: | us-gaap_RatioOfIndebtednessToNetCapital1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | instant

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- Details
Name: | us-gaap_DebtInstrumentAxis=ema_SyndicatedCreditFacilitiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |