Mortgage-Backed Securities Narrative (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2015	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Weighted Average Holdings	$ 1,328	$ 1,363

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

No definition available.

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