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Mortgage-Backed Securities Narrative (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Weighted Average Holdings	$ 1,225	$ 1,323

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- Definition

Weighted average holdings for the period, based on amortized cost.

+ References

No definition available.

+ Details

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Namespace Prefix:	earn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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- References

No definition available.

+ Details

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