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Long Term Debt (Debt Covenants) (Details) - Syndicated Credit Facilities [Member]	Dec. 31, 2017
Debt Instrument [Line Items]
Debt Instrument Covenant Requirement	0.7
Ratio of Indebtedness to Net Capital	0.61

X

- 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

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

X

- 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

X

- Details

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