Summary of Significant Accounting Policies (Residential Whole Loans) (Details)	6 Months Ended
Jun. 30, 2018
Accounting Policies [Abstract]
Number of days considered to classify loans delinquent	60 days
LTV ratio percentage (or less than)	80.00%

The LTV of which the Company has assessed its residential whole loans at carrying value will have a higher likelihood of default than newly originated mortgage loans

No definition available.

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Namespace Prefix:	mfa_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration