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Long Term Debt (Debt Covenants) (Details)	Dec. 31, 2019
Syndicated credit facilities
Debt Instrument [Line Items]
Debt to capital ratio	0.6

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

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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

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

X

- Details

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