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Derivative Instruments Schedule of Swaptions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2018
Derivative [Line Items]
Derivative, Fair Value, Net	$ 6,929	$ 22,716
Interest Rate Swaption [Member] | Fixed Payer Swaption [Member]
Derivative [Line Items]
Derivative, Fair Value, Net	181
Derivative notional	$ 10,000
Maturity term of asset underlying derivative contract	10 years
Derivative, Average Remaining Maturity	7 months
Derivative, Swaption Interest Rate	2.40%

X

- Definition

Term to maturity of the reference asset underlying the derivative contract.

+ References

No definition available.

+ Details

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Namespace Prefix:	earn_
Data Type:	xbrli:durationItemType
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Period Type:	duration

X

- Definition

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

+ Details

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Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Average remaining period until maturity of the derivative contract, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
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Period Type:	duration

X

- Definition

Fair value of the assets less the liabilities of a derivative or group of derivatives.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13433-108611

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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

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Data Type:	xbrli:stringItemType
Balance Type:	na
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X

- Definition

Fixed interest rate that will be received or paid upon exercise of the interest rate swaption contract.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Details

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Namespace Prefix:
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X

- Details

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